You can clean pot out of your system in like 3 days by drinking a shitload of water (I drank about 10-15 bottles of water each day) and exercising on a regular basis (I played a lot of basketball).

Or you could go to your local pipe shop and ask them for Detox. It is a bottle of stuff that flushes your system. It's to be used the day of your drug test (read the full instructions on the bottle or it won't work right). It costs around $20. There's also some detox pills that I've heard of.

If all else fails, try and get somebody clean to piss for you.