you're going to want to drink as much water as you can, then do a lot of physical exercise to sweat. basically what you need to do is filter your blood so that when you take the piss that they test, there won't be any waste left in the urine. you might also want to keep a half eaten poppy seed bagel in your backpack so if you fail by a little, you might be able to trick them with "i was eating a bagel."