If you keep desert species, they will require a humid hide. A humid hide is simply a plastic box that is filled with damp/wet moss, paper towels, or other mold-resistant bedding. This should be changed at least once a week, and checked daily to make sure it is still damp. An easy way to made a humid hide is to take a deli cup, tupperware-style box, or margarine tub of appropriate size, and cut a round hole in lid. This way, you can place the bedding inside, snap on the lid, and the animal can enter and exit the box through the hole in the top. This prevents animals from digging and dragging the bedding outside of the box. This method works great for virtually any species. Humid hides can also be used for animals which are not desert species, but may be having problems during shedding time--corn snakes, ball pythons, and boas will appreciate these boxes.
If you keep a lot of humidity-loving species, your easiest option may be to simply humidify the entire room. Purchase a good quality humidifier, and keep it clean and filled--simply set it up in the room with your herps. This is an excellent option if you keep chameleons or crested geckos, which do very well with open mesh cages, but require a high humidity level.
If this is not an option, or if your collection is in one room but consists of mixed desert and temperate/rainforest species, using an in-cage humidifier, misting system, or fogging system is recommended. You can cover part of the screen with plastic, as well--but keep in mind, many humidity-loving species also need good air flow. An aquarium air pump can be used to introduce fresh air to a tank, and may help with this problem. Keep everything scrupulously clean, as a closed, humid environment invites the growth of bacteria and molds which may harm your animal.
Use a humidistat for any species which has humidity requirements that exceed Cheyenne's average. Lack of humidity can cause respiratory problems and shedding and skin problems, so it's very important to make sure that humidity levels are correct. Many species can tolerate humidity levels lower than those recommended (for example, species recommended to have 80% humidity can often do well at 60%), but when they shed, they will need the humidity to be raised. Retained skin from a bad shed can cause skin infections, and even more dangerously, it can act like a tourniquet on toes and tail-tips, causing these extremities to die and fall off. In snakes, retained eyecaps can cause infection and lead to blindness. Take improper shedding very seriously, as it can become a very serious issue, and monitor your humidity levels carefully.