I was faced with this problem to after my Ball python had a problem shedding last year. The solution is quite simple.
It is really quite difficult to keep a ball pythons entire vivarium at the correct humidity, with a source of heat at one end of the tank any attampt to spray the enclosure fails as the water is quickly evaporated. I found the perfect solution to this was to look at the ball pythons natural habitat and try my best to recreate this. They generally live in abandoned termite mounds or underground in abandoned gerbil/mouse warrens. Underground it is easy for humid air to remain more or less constantly humid.
At the cold end of my snakes tank, I filled an old plastic icecream tub with moss that I got from a reptile shop (there are alternative things you can use but i'm not sure what they are). I then cut a hole in one corner of the lid large enough to provide access for the snake and a smaller hole at the other end for ventilation. Every day or so I spray the moss with water and fluff it up a bit to stop it from completely drying out. Then I simply cover the top with substrate and a bit of wood or whatever so it is dark inside. This recreates the underground & secure habitat that they like in the wild. My snake loves it and is often in and out of his tub (this is only one hide that i have in the vivarium, there are three all in all).
I never have a problem with him shedding any more and it has increased the overall humidity of the tank. I garuntee this is the best solution to the problem of humidity.