You are in luck! Terrans have a long history of invading each-other in glorious bouts of violent imperialism. If you want to know what works on them, simply look at what worked.
Copy the methods of successful empires in Earth history
This answer already describes the methods of the Soviet Union. This is a good place to start, as its rule went unchallenged (until its implosion, but we are not concerned about the fate of the whole empire here).
The British Empire was possibly the largest one in Earth history, and its control of oceans can help as parallel for your control of space. Notable things they did that worked:
Foster a local elite. The easiest way is to take people already in a position of power, and make them rule the others for you. Those who don't want to will be eliminated, you will find volunteers to replace them if needed. Use your power to keep them in power and in your debt, and pay them well enough for them to stand head and shoulders above the rabble.
From there, create a pyramid class system. This is very important, all successful oppressive empires were built on it. In its simplest form, you have your Citizens with the highest status, the local Citizens - second-class, but ruling everyone else for you, the oppressed Rabble, and some even more oppressed class so the Rabble has demotion to fear and someone to feel superior to. But really successful ones tend to use a more complex system: more classes, but also competing classes. So not only individual will compete with each-other for your good graces and the privileges going with it, but entire groups will also fight each-other to try and get higher in the pyramid by stepping on other people's faces. While everyone fight each-other to be your lackey, uniting against you will be impossible. Note that this isn't only in the upper levels of the pyramid. Even at the lower levels, people will fight each-other to not be at the bottom level.
Which leads us to the next point: foster hatreds. Cultural, racial, religious, geographic, national, the more the merrier! Humans have an instinctive "us vs them" mentality, use this to keep them divided. As the distant overlord, they may resent you but their carefully nurtured hatred of each-other will always be stronger - and it is self-sustaining! This will be one of your most powerful tools to keep the pyramid scheme working. And this helps shift the blame of your oppression to each-other, especially if you use them as local occupation peace-keeping forces.
Remember, small, local genocides are OK if the need arise, as long as you don't overdo it.
And of course, pervert local institutions to your own ends. If there is a caste system, turn the ruler/ruled/you-foul-my-air-miserable-wretch knob to eleven. If there are religious institutions, make sure the (surviving) high priests are now using a message compatible with your rule, or diffuse hatred of other groups, which is almost as good. If there are local administrations or hierarchies, take them over, etc...
An old trick, cultural domination is also important: use your control of media for bringing your culture, and keep the message that it is superior to their own primitive local cultures. Send the children of the local elites local Imperial schools, and to universities on your core worlds, to be formed alongside your own ruling Citizens. They will never be of equal status, of course, but they will think they are part of your empire, and be trained to think as such.
Another old trick: form local soldiers and send them fight/occupy different worlds. That way, not only you get more (and more sacrifiable) soldiers, but you get them to fight for you, thinking themselves as your soldiers, even going as far as sacrificing their lives for the honor of protecting your oppressive rule.
Now, those cunning humans have come up with new, interesting systems of oppression.
You can go for 1984, for example, if you are a more hands-on kind of oppressor. You need to build a (or rather, several) totalitarian system using hatred, systematic rewriting of reality through the media, thought police and eternal war to keep them in check.
You can also go for the more modern Social Credit system that one government is kind enough to be experimenting on their own population for you. You can see this as an amelioration of the 1984 system, using the wonders of mass communication and mass data treatment technologies. Possibly the most efficient thing they ever came up with, though experimental trials are still running at the moment of writing this, and it may not work everywhere on Earth due to lack of infrastructure.
With that, in one or two generations, Earth should be firmly in your grasp. There may be a few pockets of unrest or even outright rebellion here and there, but you can use that to help the "hate-each-other-and-compete-for-your-favours" part of the scheme, by sending opposed groups fight them off. And if it degenerates into an all-out uprising, well, you still have orbital control. Use your elite forces, air superiority and orbital strike platforms to protect your Imperial districts, negotiate with the many groups that hate each-other more than you, and they will spend more time horribly killing each-other than killing you. You may loose a few Imperial Citizens, but heh, you can always bring more to replace them. At worst, you can re-invade the planet.
But what if there is a really, really bad rebellion?
If something went very, very wrong, for example psyops teams from a rival empire have sown organized rebellion, this may not be quite enough. Fortunately, humans came up and have somewhat refined another method for quelling rebellions: the Terror.
Now, this can take several useful forms, the original one being the political Terror. But we are talking here about the military form, used to hit a population so hard it cannot physically sustain an insurgency anymore.
This is rather simple, really, but you have to do it right. First, hit what allows populations to sustain local insurgent forces: hospitals, urban concentrations, food production... Then, make sure you commit atrocities on local populations while fighting insurgents. Here again, having local groups hating each-other is especially useful. Then, use widespread torture. Not as an information-gathering tool (though that's one of the best disguises), as it is terrible at this job. But it is one of the most efficient tools to terrorize a population.
And never forget: hit the less extremist group first. You want to escalate the situation, and make sure that the population understands that it has a choice between your admittedly horrible but more or less stable rule, and the even more horrible and violent rule of hate-driven fanatics exposing them to atrocities from both your forces and said fanatics. Be the lesser evil by making sure you destroy lesser evils than you first. And don't worry, surviving humans are easy to escalate and join extremists (including your forces) when caught in that.
But only use it if things really go bad, this will put a dent in the value of the planet. Having a few zones like that in your empire may be useful to train soldiers - and atrocities committed in your name against local civilians a cheap way to build esprit de corps. Not as good as modern training, and possibly counter-productive for your best troops, but cheap and good enough if you need cannon fodder. But you may want to avoid those zones if you can, they can be quite a bother to sort out in the long term.
Note that those human empires tend to fall apart long-term for the same reasons that make them stable short-term: your top of the pyramid, the ruling class on your homeworlds, get used to the resources they get from the peripheries and lower classes, but exploitation of provinces tend to be unsustainable over generations (because no-one will make the effort to collect less to make it sustainable over longer than they are here in the first place). As such, at some point peripheries have been exploited to the ground, and cannot produce more.
At this point, you have to conquer more, as your core ruling class will turn on you if you don't keep their funding at the same level or more. And at some point, the empire either runs out of places to conquer or become too big to manage.
Unless something else topple them violently before, this is generally how human empires fall apart. But maybe you'll do better, who knows?