Ok, Tank Queen to the rescue once more.

To keep humidity stable in a tank, you have to have the screen top covered mostly with either foil (I have a sticky picture tutorial in the caging forum) or plexiglass. Wood, plastic wrap, and towels do not really do the job as well as Plexi and foil.

Once the top is covered, you have to make sure that you don't have any lamps closer than 8" to the top (they suck the moisture right out). Having fake greenery in the tank helps too, as you can mist that and not get the bedding wet.