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, on the low end, about 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. Bamboo can be a good sturdy climbing material,
and can often be found in craft stores, home improvement stores, and even some department 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 often using their heads/faces as "shovels," which means some
substrate may be licked off or swallowed. Never use non-organic soils, or soils with pesticides or
fertilizers added, as these chemicals will harm your geckos. Avoid large-particle bedding like bark chips, as the pieces may be ingested and are very difficult for
an animal as small as a crestie to pass. 
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.
You should consider not housing males together, as they can have a tendency to fight with each other (though in some circumstances, without a female present, some males have been noted to get along). 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. As with adults, keep in mind that even juveniles can squabble, and may especially bite a cagemate's tail, so be prepared to separate them if you see this happening. 
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.
Crested geckos are omnivorous by nature; they enjoy mashed fruit items as well as live insect foods. Up until
recently, many crested gecko breeders swore by using fruit baby foods supplemented with necessary vitamins
and minerals. However, recently, Allen Repashy, a reknowned crested gecko breeder and researcher, has formulated
a diet which has taken all of the guesswork out of feeding crested geckos. Now available in a great variety of
flavors, these pre-balanced powdered diets (to which you add water) have worked out wonderfully for my geckos,
and they all seem to enjoy the food. The food is sold in many pet store chains in small cylinder containers under the name "Crested Gecko Diet,"
and it can be found at places like Pangea under the name "Repashy Superfoods." Each of my adult geckos gets about a half tablespoon of this food every
other night.
My feeding schedule is approximately as follows:
Monday: Feed Crested Gecko Diet (approx. 1/2 tablespoon per adult), and mist geckos.
Tuesday: Leave old CGD in enclosure and mist geckos.
Wednesday: Replace old CGD with fresh CGD, mist geckos.
Thursday: Leave old CGD in enclosure and mist geckos.
Friday: Replace old CGD with fresh CGD, mist geckos.
Saturday: Leave old CGD in enclosure and mist geckos.
Sunday: Replace old CGD with fresh CGD, mist geckos.
Monday: Leave old CGD in enclosure and mist geckos. Etc.
Once a week or once every other week, I will replace one CGD feeding with live food (I currently use Blaptica dubia roaches, crickets, or Calci-worms/Phoenix worms). Live food is always dusted with proper supplements and gutloaded as appropriate.
Also, twice a month for a treat, my geckos get about a half tablespoon of a "Super Mixture" recipe which is from Sarah Milroy, or I blend up other crestie-safe fruits like papaya, mango, or blueberries.
If you are still thinking of feeding your crested gecko a baby food diet, please visit Dragontown Reptiles and meet Bubba. This animal had to be rehabilitated and will never be 100% "normal" due to his past.
It is believed by many that citrus fruit should be avoided.
Some of my geckos just plain don't care for feeder insects - they won't look twice at a cricket or roach even
if it's crawling on their snout. But the ones who enjoy them receive about five appropriately-sized crickets, or two or three roaches, once every week
or two. Feeder insects should be gutloaded with proper foods and they can be dusted with a mineral supplement. A feeder insect
should not be wider than the space between a gecko's eyes.
It is important to note that many vitamin/mineral dusting supplements available for reptile keepers
contains the vitamin D3. D3 is used by the body to process calcium in the presence of sunlight/UVB,
and because many ciliatus keepers do not use UVB lighting in their enclosures, some believe it is possible to overload
a crested gecko with D3. If you are making use of Repashy's Crested Gecko Diet or Superfood which already
has a proper amount of D3 in it, it can be a good idea to avoid excessive amounts of D3 when dusting your feeder insects. Repashy also makes available several supplement dusts which can be used to help your cresties keep the proper amount of D3 in their bodies; you can pick them up along with the balanced diet at Pangea's Products Page.
Catching your own feeder insects from outside is not recommended; outside bugs may have parasites, or they could have walked through or eaten insecticides or other chemicals which could potentially poison your gecko. Purchase your feeders from a trusted source, or raise your own from a trusted source's stock.
There is some debate as to whether the high chitin (hard outer "skin") levels in mealworms and their relatives make for poor feeding for crested geckos, due to concerns over intestinal impaction (which can lead to death). It is recommended by some that mealworms be avoided as a food item for cresties. Since they are a bit difficult to gutload with good food, I personally avoid them.
Adult crested geckos should be misted thoroughly in the evening; the humidity will help with shedding, and they
will often drink water from available leaves. I also like to leave a shallow dish (often a peanut butter jar
lid or the like) of water in the enclosure for adult geckos, and a very shallow dish (lip-gloss-lid-sized
container) for very small geckos.
Related Video: Crested Gecko Eyes While Eating