You know what I have found to do the trick quite amazingly, are those candle heating pad things... they look kind of like a cup coaster of sort, but are electric and plug into the wall. Made for heating up at a lower temp to place a glass jar candle on so you don't have to light the candle or whatever. Anyway, you put your bong water in a jar that's about as round as the surface of one of these and just place it on there....it takes a decent amount of time, but if you just let it sit there for some odd amount of hours (depending on the amount of bong water you have saved up), it very slowly evaporates all the water out of it. Much more efficient than putting it on the stove, in the oven, or the microwave technique.... but if you want that shit fast, microwave will get it there quickest, but you have stop and stir it every time you see it begin to bubble up.