Wet towel works but is a pita because you have to continue to wet it. Best way to get good, consistent humidity is to mostly seal the enclosure and use a good substrate, I like reptichip. If you go this route you rarely need to do anything for humidity, maybe spray down the substrate occasionally as needed. I found the easiest way to seal the enclosure with a screen top is to cut a piece of plexiglass to the size of your screen top, then you can cut a circle out where your heat lamp goes if you use one or just drill some holes in it for ventilation. It doesn't take much to vent and the less ventilation you have, the more humid it will stay.