I heat mine with two heating pads, one under and one on the back. I have a 19x19x30 enclosure though. I put one on back to give them a warm spot. They dont use it very often, but it gives me peice of mind knowing they have some where to warm up if they need to.
I lower the temps a few degrees at night 76ish.

I use eco earth for substrate with a nice layer of moss over it. Eco earth holds moisture for a longer period of time, but moss doesnt stick to them like eco earth does. Using substrate like that in a high humidity enclosure, you need to replace substrate often. Its easy for mold and bacteria to grow.

An easier albiet less appeasing to the eye aproach is paper towel. You dont get the same humidity raise, but it makes for a more steril enviroment.