Instead of misting the tank, why dont you try giving your snake the choice of a humidity zone. Take a small tupperware container with the lid on, and drill a hole just a little bigger than the largest part of it's body. Line the bottom with moist paper towels and set it in the warmest part of the tank. This will give your snake the option of having a humid spot to go to when it wants to. Believe me, you and I could speculate all day long about what is the proper amount of humidity it needs, but ultimately only the snake knows best!
BTW, how long have you had this snake, and how big is he/she? What does it's habitat consist of? How is it heated?
How are it's feeding habits?
-Ryan