What are you using as a lid for the enclosure? If it is a full screen top, you'll lose a lot of humidity that way...
It wouldn't hurt to include a humid hide box... What I do is take a container that the snake will fit in and cut a round access hole in the lid. Then I put a good fist full of paper towels in and soak them in water until they have almost absorbed all they can. Then, put the lid on and keep the box on the heat source. Now the snake can get as much humidity as it wants and the water won't evaporate too fast.
You could also use a hand towel, sphagnum (sp?) moss or any other moisture holding medium.
There may be better methods out there, but this is what I've come up with and it works for me.