Lots of niacin, vinegar, water. You can buy special drinks online at just about any store that will guarentee you pass. They detox your system in less than 24 hours.
If you don't have enough time, here's a fool proof method.
Take some comet bathroom cleaner (green bottle, powder)
place a dime sized portion of Comet in the middle of 1 square of toilet paper. Wrap the toilet paper up, tear off the leftover portion, swallow with as much water as you can possibly drink for 20 minutes and you'll be pissing clean in no time.