Before we get to the orbits of planets, we first need to determine the most important variable in your system: what is the size and age of your star? Obviously, habitable zones around stars depend primarily on the heat emitted by the star. This, in turn is dependent on the size/mass of the star and also its age.
For the sake of convenience, lets say we have a main sequence star at the center of our hypothetical solar system. For those who don't know, a main sequence star is one which is fusing hydrogen into helium in its core. This means, we are excluding stars like red giants, white dwarfs, neutron stars and other such types.
Out of several different types of main sequence stars, I would prefer keeping a B type star at the center of our solar system. They are 2 to 6 times larger than the sun and have surface temperatures ranging from 10,000 to 30,000 kelvin (as compared to 5000 kelvin for the sun).
The advantage of this excessive heat generation is the habitable zone will be all the more wider than the sun's habitable zone. For convenience sake, we would pick a star which is at the lower end of B type. That is, one that has a mass of nearly 2 solar masses and a surface temperature of 10,000 K.
Habitable Zone Of Our Sun
Before we get to the habitable zone of our B type star, a word about the habitable zone of our own sun would be helpful. This wikipedia article (which is quite interesting btw, unlike most wiki articles) states that the habitable zone around our sun extends from Venus' outer limit of orbit (109 million km) to as far as the inner side of the asteroid belt!
Habitable Zone Of A B-Type Star
While I don't know with absolute certainty at which distance from our B type star, its habitable zone will begin, it should be safe to assume that distances of 1 AU to 4 AU will fall in the habitable disc.
Planetary Distances And Composition
Of course you would want to keep the planet at the inner edge (1.2 AU) to be a small, Mars-mass object with relatively thin atmosphere rich in oxygen. Greenhouse gases as less as possible and lesser water content on the planet so that a runaway greenhouse effect has a chance to be reversed naturally. Also, make sure to keep the planet's core active (for a strong magnetic field) but make sure that there is almost no volcanism on the planet, as that may likely trigger runaway greenhouse effect.
The main risks for losing habitability for this planet would be the loss of atmosphere through solar wind. A strong magnetic field would be critical in keeping this from happening. Also, considering that this planet is the innermost of all the habitable planets in this system and the stellar type is B, you would want a thick layer of ozone to shield it from the deadly UV rays. Also, solar flares and magnetic storms would be potentially deadly.
In the middle (2.1 AU) you would want to place an Earth-sized planet with earthly atmospheric density and composition. A small proportion of greenhouse gases. Some volcanism and 55-65% water covered area.
This planet, being in the center of the habitable zone, will be the one with least risks. Even if it temporarily loses its habitability status due to a gigantic meteorite collision, it will naturally regain its habitability within a couple million years or so.
On the outer edge (3.8 AU) you would want to keep a super-Earth. A planet with 3 Earth-masses, thick atmosphere, rich in carbon dioxide and lots of volcanic activity. You would also want to have many organisms producing methane, as it would help in the greenhouse effect.
The hazards for this planet would be any perturbation in its methane production chain or too much increase of flora on the planet, that could trap most of the carbon dioxide from the atmosphere. Also, any snowball events would have a dangerous tendency to be perpetual, ending its habitability status.
I strongly recommend you to read this article. I have found it to be very knowledgeable and written in very simple language.