You might try some additives to your sample, if at all possible, bleach crystals (or liquid bleach, though small amounts so the sample doesn't smell like bleach). Your best bet, though, is to wake up early before the test (like at 7 or so) and begin drinking water. Keep pushing fluids, but if you begin to feel lightheaded, dizzy, or if you begin cramping, STOP. You've had enough. Take the pills a few hours before, so they have time to metabolize. Hopefully, your sample will be dilute enough to test negitive. Otherwise, perhaps smuggling in a clean sample???

A word of caution on too much water: during the last Boston marathon, a woman actually died from drinking too much water, because she way overcompensated for what she thought she was losing. The latest studies show that some of us actually need to drink a lot less than the old addage of "8 glasses a day," which means that you can put yourself in danger by drinking too much. This puts a lot of stress on the kidneys, and can put your electrolytes all out of balance, even causing things like heart attacks in extreme cases. If you are going to load up on water before a test, please, please try to eat something salty and have a banana before you start. You might also drink Gatorade or some other sports drink. Also, listen to your body. It may be uncomfortable peeing all the time, but it SHOULD NOT hurt.