Don't use towels - 1) they suck up more moisture than they give off, and 2) they harbor bacteria like mad.
The best way to keep good humidity in a tank is to cover the screen top with either plexiglass fitted to the top, or with the foil treatment. These are the two best methods that get good steady results.
I have a picture tutorial on treating a screen with foil and can PM it to anyone who asks.
**Adriana - White 'N Nerdy!**
1.0 BP 'SunSpot', 0.1 Corn 'Freya', 1.0 IJ BTS 'Topaz', 1.0 ND bunny 'Licorice'