Yep, if you have a screen lid, try covering most of it as Smulkin said (though you don't really have to be so precise as to make triangles -- rectangles and irregular shapes work just fine, too ). Also, in addition to size and placement of water dish playing a role in humidity, material of water dish can make a difference as well. If you have trouble keeping the humidity high enough after covering the top (I doubt it) or for when you want a boost for shedding, try putting in a glazed ceramic soup bowl from the dollar store or WalMart as your water dish. Ceramic, glass, and most types of stone conduct heat better than pvc and some other plastics commonly used for water dishes.