first of all, if you know you have a drug test in advance, and still fail it, you have issues.
but let's say you have 5 days. Weed is the hardest of any drug to get out your system (I can do an eightball of coke on thursday, take a drug test monday and not worry it's in my system, assuming I'm not an every day user)
First of all, Take 4 asprin 3 hours before the test. Asrpin is the only thing that works as a screen again drugs.
Secondly, when you piss, always give an "mid stream" sample... the beginning of the stream is always the dirtiest
what else... drinking all that water days before the test does no good, it just makes you feel uncomofrtable... you need to be exercising, that's about it.
drink 8 to 10 glasses of water (until your pee) is white before the drug test, right before, and take a vitamin B to add color back to your piss... make that 2. the idea is to dulude your pee. There has to be a certain amount of THC in your piss to come back as a postive, deluding your piss well... it deludes it.
If you have a couple of days, you can go to walgreens and they make a cleanser you can buy.
Also, at walgreens, you can by an at home test for $15 bucks.. take it at home before you go. if you fail the test at home, don't be a dumbass and go take the test at the lab.