This weekend, starting at breakfast, load her up on sweet drinks and salty snacks. The snacks will make her thirsty but the sweet drinks won't quench the thirst. Keep her in just undies or a shirt all day and don't go anywhere. Make her responsible for cleaning up any messes she makes - not as punishment but as a matter-of-fact "clean up the mess you made" way.
After she gets the idea, monitor how much she drinks. She may very well be limiting how much she drinks so she doesn't have to pee much and that's not healthy.
at 11:49 AM on Feb. 5, 2009