I would suggest that you go to a local health food store and request some detoxificaton tea. The good ones offer a money back or double money back guarantee.
Follow the directions to the letter!
Usually the mechanism here is that you have to abstain for 2-3 days before hand, then in the day before the test, you make a couple gallons of this herbal tea with the usual cleansing herbs like rose hips and goldenseal. You drink all of this tea and as much other water as you can and pee as many times as you can. Then, about an hour or so before the test you take a couple of capsules that restore creatine and yellow color to your urine so it it's difficult to tell that you're trying to beat the test.
Also, it's more expensive, but you can actually buy freeze dried, clean pee that you add water to and heat to submit. If they watch, you can even buy an *ahem* lifelike looking implement to distribute your sample.
I'm sure that you can also find suitable sources for this on www.hightimes.com or do a google search.