I agree with Topspin -- good advice.
Then again, how much is too much? When I was having kidney problems I was lucky enough to get an appointment with Dr. Chan-Yan, a noted Canadian endocrinologist. He gave me this little nugget:
Very few people drink enough water. You should actually drink so much water before bed that you need to get up and pee at once or even twice through the night. If everyone did that, many health problems could be diminished.
I follow this advice as much as possible. Luckily my GF is a sound sleeper!