What I read and worked for me was that they look at a number of things in a piss test. Color, creatine levels, specific gravity. If you drink lots of water then you'll likely have lower levels of all of the above including the remanants of the cannabis. Drug testers know this and can get you to resubmit if things don't look legit and think your trying to cheat by drowning your piss with water. So you drink Gatorade to increase the density/gravity of your piss ie salts. Take a b12 supplement to add colour to your piss. Also a creatine supplement to mask low levels that you'd have if you just drank a gallon of water etc. Don't fill the cup with the start or finish, but rather the middle of your piss. If you have time in advance then go onto ebay and buy a few drug test kits as you might not need to do the above. LOL, I still remember driving into town for the test, I had to stop twice to do a piss by the side of the road from all the fluids I had drunk. It was a proper lab test and I passed - I was a non stop daily smoker at the time.
