Rav Yisroel Reisman, in his weekly Thursday Parsha shiur last week, said the following:
"You don't want to oversleep, there's one sure way to make sure you don't oversleep. Before you go to sleep at night, you drink half a gallon of water. Drink half a gallon of water, three quarters of a gallon, a gallon would be the best, and then you're guaranteed to get out of bed on time in the morning."