I've totally solved my humidity problem! Here's my secret:
I have a 40 gallon tank with a screen lid, some of these tips will work well for a tub.
Use repti bark (I love that stuff, it looks great and works very well.)
Get some zoo med spagnum moss (it's like 7 bucks) and soak the moss completely, put it in a small plastic tuppawear container with a lid, and cut holes on the top of the lid, then put that in the tub. (This moss works like a charm btw)
And I cover most of the top of the tank screen lid with wet towels. My humidity easily stays at a constant percentage of 57-60% and I can easily raise my humidity for shedding.
Hope this works :~) good luck!