go to your local hippie store, say, "hi. i need something that'll make me pass a drug test." they will give you the shit.
also, drink water, and generally be a very active person. if you sit around, do nothing, and smoke budzz all day, then you won't pass it. skateboarding or trampolining don't count, either.
i'm on an elite cycling team, THC passes through our systems in a day. so just go out there and run and stuff. but first check with your local hippie store.