Crested geckos thrive at temperatures between 72 and 80 degrees Farenheit, general room temperature for many homes.
While they deal fairly well if the
temperature dips a few degrees below 72, you will want to avoid temperatures much higher than 80. The excessive
heat can cause stress in the animal, which could put it off of its food, and possibly eventually kill it. If you
have no means of cooling your home to below 80 in the hot months, a crested gecko is not a good choice for you.
Using a digital thermometer and hygrometer to monitor temperatures and humidity is a good idea.
It is also a good idea to have a backup plan in place in the off-chance that your power goes out on a very hot day and you can not run your air conditioner. You can put the gecko in a watertight enclosure in a bathtub with a few inches of cold water inside the tub, or you could take a couple of bottles of ice and put them inside the enclosure.
A single adult gecko should be given at the very least 20 gallons of space; as they are arboreal, it is preferable
that the enclosure be taller than it is longer, though larger long tanks can be used too as long as there
are a lot of good climbing materials. Set up good thick branches which can support
the gecko's weight (if you acquire branches from outside, they should be carefully baked in an oven, and frozen
for 24 hours if possible, to attempt to kill off any infesting bugs. For this reason, I prefer store-bought
bamboo sticks, which can be inexpensively found at some home improvement stores). 
With any kind of setup, it is important to keep in mind that no matter what kind of substrate you choose to use, your gecko may have a risk of ingesting it, and impaction can occur. While this is less likely to occur if you use something like paper towels, the risk is increased if the gecko lunges for live prey and snags a tooth on the paper. If you have a smaller collection of crested geckos, it could be worth your time to feed the geckos their live prey items in a separate container which does not have a substrate.
For a simple setup, the
floor can be covered in whole paper towels, and fake wide-leafed plants can be used for cover. Adding coconut hides,
decorative pots with a hole carved out, or other items for the geckos to nap inside can help them feel more
secure during daylight hours. Droppings should be removed daily, and the paper towel substrate should be
changed at least weekly.
For a naturalistic setup, some people recommend using coco bedding, some use eco-earth, some use leaf litter, some use moss, some use a mix of some of these things. I recommend doing a lot of research before settling on a naturalistic substrate, as crested geckos - especially nesting females - will dig into the soil and "taste" it to see if it's right for egg-laying, so you need to be aware of what will possibly be going into your gecko's stomach. One word of advice - never use non-organic soils, or soils with pesticides or fertilizers added, as these chemicals will harm your geckos.
A good hardy plant for naturalistic setups is the pothos - they're very
easy to keep, eventually grow in a viney manner, and they have flat wide leaves which are perfect for
the geckos to walk on.
Adult and pre-adult males should not be housed together, as they generally will fight with each other. Females can generally be housed together, the number dependent upon the amount of space and number of hides you have available, but please continue to keep an eye on females as there is a possibility of them fighting with each other as well. Males showing a hemipenile bulge should not be housed with non-mature females, to prevent breeding at an immature age.
Smaller juveniles and hatchlings should be housed in smaller enclosures, so that they may more readily find
their food sources. Two hatchlings can fare well in a small Kritter-Keeper-type case, as long as there are
leaves and a small branch or two to climb on.
It is important to give your gecko a rotating day-and-night cycle; they should have at least 8 hours of darkness (preferably more) as well as light. Because crested geckos are nocturnal, there is still some debate as to whether UVB-type lighting is required; during the day when the UVB would be turned on, many crested geckos do all they can to hide under leaves and inside hides, where the light would not affect them. Because of this, many breeders do not make use of UVB lighting. If you choose not to make use of UVB lighting, it is important to at least make sure your geckos get a cycle of light through a window or florescent lighting. Keep yourself informed about the uses of UVB lighting with crested geckos by searching out forums and publications about their keeping.