Yeah I've had to deal with this problem recently - its a pain in the ass.

Those detox drinks apparantly don't work any more because the tests now look for additive agents or anything out of the ordinary. If you drink too much water (which is a method some people use), they'll know it and you'll fail. Apparantly the only real method to passing it if you're using is to take in someone elses pee -- but then the problem with this is that it's gotta be warm -- you can't hand the lady on the other side of the door cold piss. Either that, or you can buy this.