What you wanna do is just drink tons of water. I would say about 2 gallons a day. When i got busted they put me in rehab, and one sunday i couldnt help myself ( the joint was lit by my friend, sitting next too me in the back seat) and i had a piss test on the next wednesday. I just drank 2 gallons a day, pissed a lot and passed. Drinking that much water is also really healthy, as is pissing a lot.