Crested geckos over 5 grams or so will do fine at just about any reasonable room temperature, but hatchlings need to be kept in the low 70's. If you don't follow that bit of advice, I can all but guarantee you'll learn the hard way. I cannot emphasize it enough...keep your hatchlings cool!
In my facility, the daytime high is about 74-76 degrees in the summer and 70-72 in the winter.
A popular misunderstanding about humidity and reptiles is that the humidity needs to never drop below a set percentage. This is not true. You actually want the cage to dry out and humidity to bottom out at whatever it is in your reptile room, creating a humidity cycle. We like to keep the room around 60% humidity and then mist hatchlings as necessary.
If the humidity stays too high for too long, bacteria and fungus will start to grow, creating an unhealthy environment. This is why it's important to have plenty of ventilation and to let the cage dry out every day. I typically spray cages at night and let them dry out as the day gets warmer. As a loose rule of thumb, I try to spray the cages so that the water has mostly evaporated within 6-12 hours.
While Crested Geckos do not need constantly high humidity, babies dehydrate more easily and should be misted more regularly to make sure that they don't have shedding problems. On the other hand, I've noticed that reptiles that are kept too wet for too long can have shedding problems as well, so you still want baby cages to dry out periodically.
The Humid Hide
In the winter, when the furnace is warming and consequently drying the air that's pumped throughout your home, it may be difficult to keep the humidity even at a moderate range...meaning you have trouble keeping the cage from being either too wet or too dry. If this is the case, you can try giving your gecko(s) a humid hide. A humid hide is essentially our way of reproducing the humid underground retreats that a lot of wild reptiles will take shelter in during dry spells.
We usually use a plastic container (like a deli cup or gladware container) with a hole cut in the lid and then we fill it almost full with the same mixture that we use for nest boxes. If you don't like the look of a plastic container in your cage, you can cover it with foliage. Another good option for filling the humid hide is New Zealand Sphagnum moss (not sphagnum peat), which is also resistent to bacterial/fungal growth. Paper towels are not a good choice here. The idea here is to find a humidity solution that doesn't require a lot of work. You want a small hide filled with a substrate that doesn't get moldy, like Sphagnum. Remember to just give the gecko enough room to fit into the hide...the less substrate in a container, the more airflow there is, and the faster it will dry out.
Again, you just want to give the gecko enough room to squeeze in...meaning maybe fill the humid hide to within an inch of the lid. One you have to remember is that the tighter the area, the more secure the gecko will feel and the more likely they will be to use it. Animals are hard wired to seek out hiding areas that are too small for predators to fit into. It's as if they instinctually know "If I can barely squeeze in here, then there's no way something big enough to eat me can get in."
How Many Geckos Per Cage?
Adult Breeding Groups
My Crested Gecko adults are housed in groups of one male and up to three females, in rack systems which house 66 quart Sterilite containers (found at Target stores). Sterilite also makes a very similar sized tub (58 quart) which is available at Wal-Mart. I prefer the Target version because its outside dimensions are almost identical, but it has 8 quarts more volume simply because it's not nearly as tapered at the bottom. More usable space is always good in my book.
Hatchlings to Juveniles
While I prefer to raise them individually these days, that's only because they tend to grow faster without competition for food. Groups of geckos will get along just fine together, provided you keep a close eye on them, they're kept in a sufficiently large cage, and they have plenty of hiding areas, food, and water
Crested geckos seem to do well in naturally planted and decorated vivaria. In the wild, they tend to hang from thin branches rather than flat against the trunk of a tree. Plants can include ficus trees, philodendrons, etc. It's important to make sure that the plant(s) you use are not toxic to reptiles and that they have been rinsed thoroughly, in order to remove any pesticides that may be present. Several keepers and breeders have had much success keeping their animals on bioactive soil substrates (a mixture of soil, sand, etc.), which require little work and theoretically never need to be replaced. Other keepers say that ground coconut fiber works well.
To be truthful, I've never really kept Crested Geckos in a naturalistic style cage for any length of time, so I don't really feel qualified to write a full section about this topic. However, there is a comprehensive section about naturalistic vivaria in the old book, Rhacodactylus: The Complete Guide to their Selection and Care, written by Philippe deVosjoli, Allen Repashy and Frank Fast.
Nest Boxes for Females
In addition to the egg crates, I provide adults with a nest box filled with a moistened peat moss/coconut husk/sand mixture. I place something heavy (like a hide box) on top, which the females seem to gravitate toward when depositing their eggs.
Nest boxes should always be placed on the coolest end of the cage, in order to keep them from drying out too quickly.
I use newspaper or paper towel as a substrate to simplify the cage cleaning process. I do not use loose substrates (like bark chips, mulch, etc.) because babies may choke by accidentally ingesting them. With adults, this is not so much of a concern. However, when given a loose substrate, adult females may deposit their eggs anywhere in the cage, making my job (collecting eggs on a daily basis) more difficult.
We recommend artificial foliage, especially for hatchling geckos, as I feel this provides them with ample cover, plenty of drinking surfaces (with regular misting), but it's still easy for them to find food/insects in their enclosure. Crested geckos are naturally inclined to "hang out" on thin branches as opposed to resting against the trunks of trees, so this has become my cage furnishing of choice for young geckos. Six foot strands of artificial vines are relatively inexpensive (Wal-Mart sells it for around $3), so it can be thrown away and replaced.
Due to the sheer volume of foliage we use, we typically soak soiled foliage in bleach water for 10 minutes (1 part bleach to 10 parts water) and then rinse it off and re-use it.
Rather than providing water bowls, I used to just spray the cages every day to provide humidity and drinking water. Now that we have a larger collection in a more humid facility, water bowls have become our method of providing drinking water, while the large humidifiers keep the humidity just high enough (remember, humidity isn't a big issue with these geckos).