I was using a uth in a long 20 with a heat lamp on the back so it would warm the glass at the top (I know it was not the best set up but I did what I could with a dumb person at petsmart) and I put a blanket over the top and that kept the humidity at around 65%.
Like I said not the best set up but the blanket helped with the humidity.
I forgot who had the sticky but there is a great write up about glass tanks. I am about to set up a plastic habitat for mine.