The old answer is "eight glasses a day."

Well, roughly. Your daily water requirement depends on your body type, your activity and the weather. If you're active, try figuring out 2/3 of you weight, then drink that much water in ounces. For example, a 175lb. man should drink 117 ounces (about 10-11 glasses) of water per day.

The best predicator, though, is thirst. If you drink enough water that you rarely feel thirsty, you're on the right track!