A year ago, feline mammalian space pirates, a remnant of a collapsed civilization, conquered earth with their advanced technology (their civilization was Kardashev type I) and built a megacity on the continent of North America. The pirates killed 90% of human population but spared the lives of the rest. The aliens want to use humans as entertainment in circuses and zoos also as sex slaves, but the survivors are apathetic, sad and angry. They are not good as entertainment, but the cats love human physical appearance. How do I make humans happy again?
Give them hope and a way out.
First, have very strict laws on what can and cannot be done to the slaves. Knowing that they cannot simply be tortured for the sadistic pleasure of their owners or simply killed for no reason would eliminate a great deal of anxiety.
Second, have an incentive for the slaves to work hard. In ancient Rome, slaves were paid very modest wages, could own property of their own, and even buy their own freedom. Have pay rates higher for the slave positions you want filled. Having them see a way to better their position would put them in a far better mood and motivate them to excel.
Third, the slaves of a more carnal nature should be given a very high status and limited period of servitude in that role. Strict laws against disfigurement and physical abuse should apply here. The exception to this should be that any slave that harms another slave would be sentenced to this role without any protections and may even be tortured to death for the pleasure of its master.
Any harm to the masters done by a slave should have as its penalty, a quick execution.
Laws should be in place for the confiscation of poorly treated slaves. Any owner not taking proper care of slaves will have slaves removed and placed with new owners.
Lastly, all working slaves should be provided with the necessities of food, shelter and clothing with the opportunity to use their wages to buy up into better conditions. Slaves would initially live in a common area but could buy their way up to private apartments and eventually private housing. Buying better conditions rather than staying in the common area would also prolong the amount of time it would take to purchase their freedom while at the same time improving morale.
Make them think they're not slaves.
A human that is a slave, and knows they're a slave, generally won't be happy, and there's not much that can be done about it. But if you can convince them they're NOT slaves, they get a bit more comfortable.
Maybe you phrase it so that you're covering all of their living expenses for this job. Hell, maybe pay them a pittance so they can buy some things for their down time. Make sure they're well fed and supplied with plenty of entertainment - Including drugs, so long as they're not going to affect their performance as entertainment.
Also, allow them to choose just what it is they do. Some might enjoy being in a circus. Some might want to be a prostitute. Some might relish the opportunity to be in a zoo and be "natural." There'll be times you'll have to say no, but if you're generally nice and willing to work with them about things - or at least appear to - then you can get away with a lot. Move the blame from yourself to, say, "Regulations" or "Corporate" or whatever. Then the Humans can blame that big negative, cloudy entity but not YOU.
Train them from birth to become pets and entertainments. Gather them in camps to train for about 6 years. Then put them out to sell to owners. Never give them the idea that things like 'society', 'human' or 'rights' apply to them. Train them to always happy, always smiles in present of the owners and think happy thoughts. Tell them that the good pet does not speak.
Kill all the old ones. They're not trainable.
After 2 generations, you have yourself a new species of absolute obedience smiling pets.
There's the Matrix model--from the movie The Matrix, in which most humans are not even aware that they are slaves...
Employment Brokers aka Slavers, who get a percentage of their selling price, but the slaves are actually unionized and there are specific rules as to their treatment, specifically geared to what they can do/what their value is. The dream is to be bought at a high enough price that you are richly maintained, and specific things are provided for you.
This gives the slaves something to strive for.
Rules could be a certain number of days off per week, a certain quality of meals, education (which may drive up their value depending), supplies, bonus monies and that sort of thing.
What makes it slavery is that they are owned and not allowed to leave permanently. Running away drives down a slave's value and means that they are not treated as well...
They might also negotiate their own terms or they can sell themselves as long as their current master gets at least what was paid for them, so an auction might go like this:
Lot 449 Contortionist, love making, and calligraphy.
Requires two days of rest, a ration of chocolates each week, three meals a day, a bottle of wine per week, theatre tickets once a month, and a gift of jewelry once a year not worth less than $300, which can be taken with them when they are sold.
Minimum bid starts at $12,000
My numbers are arbitrary of course: this is just a system I can see working (and resume may be longer and include former prestigous owners. There will still be runaways and those who are unhappy.
Make them dependent on their status as slaves, which is essentially what humans did with domesticated animals. Thus, even if you gave them their freedom, they would re-enslave themselves in order to have a master who tells them what to do.
Or, another way to put it, breed them for docility, loyalty and juvenile traits, and raise them in an environment where obeying the Master gives them a reward. Like what humans did to canis lupus
this needs to be tied in with how the slaves are treated. You ever meet someone who's never left the city? You ever meet a rich kid? you ever meet someone whose parents did every little thing for them and protected them from living? a slave can have one or two jobs they do, but as long as that's all they ever have to learn, they won't know how to take care of themselves. they shouldn't even know how to prepare food for themselves.
I doubt there's anything you could do to make them "happy", even mildly oppressed people are generally going to be upset about it.
But there are some things you can do to make them less likely to revolt...
Bread and circuses are the classic route and you already have circuses. People who are well fed and entertained are probably going to be a little more manageable.
Another approach is to further stratify your slaves into classes, so there's at least the hope of upward mobility. Note that they don't actually need to be able to significantly change their position, they just need hope, an illusion of upward mobility.
"If you work extra special hard, one day you could be the head slave. Here have a cookie. Who's a good human? You are!"
Drugs. All kinds of lovely, feel-good space uppers will make these remaining humans fun again. That seems easy, at least, but maybe temporary and have all kinds of adverse effects.
More difficult options include breeding, sophisticated training, psychological trickery, or even genetic manipulation.
Not being slaves ruled by evil dictator overlords is the usual first step. The lesson of all slavery-based human cultures is that unhappy slaves rebel.
What stops slaves rebelling is being given the opportunity for self-fulfillment. Roman gladiators, teachers, musicians, cooks and many other professions were frequently slaves; but they were valued members of society, able to contribute and rewarded for doing so. If you can give jobs to your slaves which they enjoy doing, they will not really care that they're slaves, because they're living a fulfilling life.
If the slave-owners only want entertainment in the form of circuses and sex slaves, that's less likely to give humans a fulfilling life. Some people today do find fulfillment in those jobs, but nothing like all of us. The slave-owners need to find a range of employment options for humans which includes these roles but also has other places available.
In fiction, the "Tripods" series describes a rebellion amongst humans who, like the OP's question, are kept alive primarily for entertainment.
Conversely, the final section of Grass describes a woman willingly accepting being a subordinate to a creature from another species who demonstrates that he truly cares about her. She refuses to let her pets die to save herself, which convinces this species to commit themselves to protecting humans in the same way.
Given that there is already good quality and variety of answers, I'd like to categorize them a bit and add my own thoughts. I think this may be too long for a comment, so I'll just add it as an answer (giving proper credit).
In order to answer, I'd ask myself: Do the pirates have moral issues or guilt / Is there dissension among them? Do they want slaves to behave naturally?
No moral issues and no dissension, artificial behavior
If their only concern is the slaves' perception of themselves as "happy", then merely modifying their meaning of happiness would be sufficient and most effective, as in Ngoc's answer.
A second best alternative is to create inequality. That is, make some live worse lives to make the rest value their own. This is bound to generate a lot of resistance and unhappiness. Also, given how inequality between them and the pirates will still exist, even the favored ones' happiness won't be complete. The only added benefit, compared to the previous alternative, is that it wouldn't completely erase human culture and natural behavior, at least not immediately. This is related to SJuan76's answer.
Drugs may not be a good alternative as it'll shorten their utility.
Some moral issues or possible dissension, natural behavior
If the pirates' motivation is making them happy as their owners perceive and define happiness, then improving their situation while changing their perception of themselves is probably best, as in Andon's and Robert Wm Ruedisueli's answers.
The Matrix model is not economically viable if such a technology hasn't been implemented already (note that I'm not saying "invented"). However, it can be, if taken metaphorically. This relates to the first part of Erin Thursby's answer.
Actual moral issues, natural environment
Setting them free is a real alternative. They could just simulate a coup d'état, change some "top" figures and everyone's happy. This is related to Journeyman Geek's answer.
A few things I'd like to add:
Slaves have typically (note that I'm not saying always) been used for material gain (agriculture/mining and manufacture vs. services). As with most things with humanity, a convergence of economic interest and ethics advancement was needed for its abolishment.
If they won't be used as a factor of production and they develop a symbiotic relation with the pirates, then they'd practically (statistically) be closer to pets than to slaves (they'd technically be slaves though).
If they will be used for production though, it'd be best to have them get paid as per Andon's answer. This would eliminate the risk of losing the investment due to unforseen events (assuming private property exists, of course — and seeing they're pirates, I'd assume it does).
From a narrative point of view, quick solutions would not be viable without a turn of events at hand. This may force the plot if not properly done. Given the fantasy set, perhaps a somewhat believable/natural development would be best to balance things out, though that's ultimately your decision.
In summary, my answer would be "it depends". I enjoyed reading all answers and would like to note Ngoc's answer's sociolgical component about denaturalization.
Make them afraid of the alternative
Do not kill all the other humans. Allow them to live, but just that.
While your slaves will have a not-so-lavish food (meat or fish only once a week, candies only if their masters is in good mood, etc.), but enough for them to be nourished.
Now, each time the meal is finished, have a few of them collect the scraps and dump them in the outside of your properties. Where the free humans are waiting, to feed themselves (surely a Type I civilization will have no trouble making all the edible plants and animals extinct).
Let the slaves see how the free men are starving and fighting for the slaves'scraps, and they will think that they are privileged by being slaves.
The usual approach in America has been for slaveholders to convince themselves that the slaves are already happy.
It's a fascinating topic, really. Mental disorders causing slaves to run away were posited, for no other reason a slave might runaway was always obvious. Sometimes it takes serious historical research to debunk the myth that slaves were generally happy (can't imagine how anyone would get that idea).
It's very similar to how today we blame people without a job or health insurance for not working hard enough, being in prison for committing crimes and harming people, etc.
Set them free. Seriously.
You'd have a happy, basically motivated society, that's mostly pissed at you. Your fault, but they have a lot of space to rebuild on. That'll keep them busy.
A small faction of them might actually find said felines attractive (furries?) or merely suffer from stockholm syndrome. Employ them, treat them well, and you might attract a few more.
Considering how human history is littered with "superior" nations defeated by small, motivated armies, leaving this planet alone would also be sensible, cause eventually you'd find random acts of sabotage and open insurrection.
Students of human history would probably realise that few nations of slaveholders rested easy.
Well first, the aliens shouldn't make it out to be slavery.
The easiest form of brainwashing of a slave is to make them think it's a privileged to do the work. They are concubines, not sex slaves.
The humans should be convinced to compete by grooming their beauty and developing their behavior to find a good master to take care of them. This should culminate in beauty contests for the auctions when humans become of age.
Furthermore, this eases ethical concerns by prospective feline masters by convincing them that they are granting the humans a great privileged.
Give them a purpose to live and fight for.
Make them think that they are the human resistance movement which, against all odds, fights against the alien oppression to win back control of Earth. To make it easier to control spread the resistance settlements over a bigger areas with few (and dangerous) communication and transport paths between them. This will give keep each individual cell small enough to be manageable, also you can easily pick the humans you need for your pleasure without alerting the whole population that something's up (because you basically control all communication between cells).
Install a brain shunt.
Admittedly this is a more high tech version of @fredsbend answer. ("Give them drugs") but direct brain alteration is a possibility. This would involve inserting a small wire through the pleasure center of the brain and delivering the right current. (Oddly, there isn't a pain center of the brain)
This could leave you with utterly blissed out performers and really pissed off people being bred for the circus. Get it wrong and your performers won't do anything - potentially not even eating.
From the felines perspective - none! The humans are property, and making your property more productive and happier is an obvious moral choice. Humans are, obviously, not people - they eat vegetables, aren't covered in fur - so they shouldn't get the same moral consideration.
If you have several (human) generations, don't make them happy. Make it horrible. A life of slave labor and subsistence farming and devoid of educational opportunities. Kids have got to grow up knowing that life is toil. This is your first "we're better than everyone" bonding moment that I totally stole from what I remember of the descriptions Emperor's prison planet in Dune.
You've killed most of the pet species, so you can control any rebellions that might spring up. Once morale is low enough, select certain pets that demonstrate desirable qualities. Give them titles and spoil them with treats. Appoint a few as ombudsmen or "union reps" for their fellows in specialization (lap pet vs. sex slaves). They should relate to one another as better than those who don't have titles and traits showing worthwhile breeding.
Spay/neuter your pets and keep them happy with whatever cocktail trips their trigger. Think the eldest son in Farnham's Freehold (if I recall correctly, he was either too intoxicated to notice or accepted intoxicants as a suitable substitute for his castration).
Cultivate a furry fetish amongst humans
Keep most of the earth exclusively habited by humans. Encourage people to be working long hours, but give them lots of possessions. Encourage them to enjoy their possessions rather than interactions with fellow humans, and be bowling alone. If they do interact with other humans, let it be via the internet. Soon enough, they'll stop having sex. When that happens, you start getting fetishes. And where do you get those fetishes from? The internet. When you're on the internet, no-one knows that you're a cat, secretly manipulating humans.
Keep the felines in a particular part of the earth. A place where travel is only by plane or boats, so that people can't enter or leave without your control (say Japan), and control the flow of information from that place, so no-one knows outside of Japan about the feline control of the planet. Lure those with the furry fetish to that country. Once they get there, some may agree to be prostitutes to felines, and some may not. Keep the former, and kill the latter.
Religion. Make them believe that if they live a "good life" (where good life means "work hard for your masters without any complaints") they will have a wonderful eternal life. Make them believe there's an all-loving god which will take care of them, but that he demands you follow his rules. Make them believe that any deviation means they'll be tortured in hell forever.
So, basically, the feline overlords take Christianity, and make some tiny adjustments.
Target people who feel like they need to escape; anybody in an abusive relationship will do, but runaway youth are especially good targets because they don't always know how to take care of themselves. It's not foolproof, because some of these people are very hardened by their experiences, but enough of them will be apathetic or thankful enough not to rebel.
There is a non-intrusive, non-harmful way to do things: Selection.
First of all, just make sure that not one of your slaves comes in touch with schooling. Ignorance is bliss! They will never feel the need to be free if they're cared for and kept away from each other -remember, man is a social animal, must not be given the chance to form a group.
Second, inbreed them: Any advanced sentient species knows how dangerous to mental health is inbreeding, so make sure that they are tamed by genetical means. Have that trait breeded generation after generation, and masters won't have to worry about revolts.
Third: eradicate immediately all rebellious individuals. If their streak reproduces, they can pose a problem in the future. A discrete thing, no public retaliations, so that nobody feels like 'solidarizing' with the rebel