Be careful with the rag; watch closely for mold or fungus growth. I've had damp cloths over the screen top that grow mold.
Gloryhound is right; moist hides and covering the top work great.
I'm with you; but it's 25% in my house! I have experimented with a bunch of things; covering the top helps a ton. I have also experimented with various substrates. When I tried "Eco Earth" (coconut fiber), the humidity wouldn't go below 80% for three days, and I took it out and threw it away for fear of scale rot. A very thin layer of Eco Earth on top of another substrate such as aspen or repti-bark might be beneficial, but I warn you against using Eco Earth as the only substrate. I am currently trying out repti-bark, which is much dryer than aspen and a bit better than aspen, but it is still pretty dry, even with a little bit of Eco Earth mixed in. I have yet to get cypress mulch, but I have heard wonders about it.
I find that during the dryer months in winter, I simply can't get the humidity up, no matter what I do, so I put the balls in plastic tubs with holes drilled in the sides. These work great for holding humidity.