I use a shammy, and it works great! (maybe spelt chamois?) When it dries up, it gets hard, then I just dunk it into the tank water to get it wet, squeeze the water out so it is just damp, then the water marks are a piece of cake to get rid of.
Found one at walmart for like 10 bucks, and i even had to cut it down bc it was such a big piece.
TRY IT! you wont be disappointed.
I originally bought it so i didnt have to use a crappy washcloth or paper towels every time i reached into the tank. shammies work WAY better then regular towels (thank you diving/swimming for teaching me about this cloth!)
in my opinion, its a must have.