Well the heat lamp is killing your humidity. There isn't much more than what you are already doing to keep the humidity up.
This is why many people use tubs or PVC cages, they hold humidity much better and usually do not use heat lamps to provide the proper temperatures.
Here is a thread on tubs:
http://ball-pythons.net/forums/forum...2-BP-Husbandry
And one on PVC cages:
http://ball-pythons.net/forums/showt...With-Pictures!