what kind of a drug test is it?
-dip strips or a urinalysis (sent away to a lab)?
-are you being watched by someone while you piss?

*if its a dip stick test, you can buy one of those "cover-up" drinks, not the "flushing" type of drinks, theres no way anyone could flush their system if they smoke the day before. ive used those before and the work only for dipstick tests, if you try that shit with a urinalysis, crazy ass shit will come up in the report and they'll know for sure...

*if you're not being watched while you piss, have a buddy who is clean piss in a small "sterile" plastic container or something you can fit in your pocket without anyone noticing. put it up to the heat vents before you go in to get it nice n warm, wear a hoody or something to keep your hands wrapped around the container. you get the point.

ive had many a piss tests, and these methods are tried and true.
i never got caught.
peace