The key is to drink the water throughout the day, not try and get it done and over with in a short period of time. At work, we inject a patient, and then they are supposed to drink 6 glasses of water over a 3 hour period of time for the test. I always have to stress that they drink it slowly, about a glass every half an hour, because, if I don't, one of them will try to finish it in the first 15 minutes so he/she doesn't have to worry about it. But what happens is that drinking it that fast changes the electrolyte balance in the body and they get dizzy and nauseated. We also stress that they don't drink any caffeine, since it can be very dehydrating, which is the opposite of what we are striving to accomplish.
Another thing to look at are sodas. Did you know that carbonated beverages decrease your absorption of calcium???
Marian