Okay,
Everyone touched up on all the important parts. Some things are fake....
THC does stick to lipids. So unless you do a shit-ton of running your not going to get it out of your system, all you can do is mask it. I had to take a test, had 3 days notice. I did a ton of research, this was the same time my friend failed two drug tests within two weeks before.
(Shouldn't smoke when job hunting but thats my opinion)
When you find out that you have to take a test, don't smoke. Eat lean foods to prevent any residual THC in blood stream to bond to new fat cells. Drink as much water as possible per day (+1 gallons). Now for the important part.
When they test your urine they are testing (besides drugs) for 1) creatin (possible misspelling, its not the muscle building stuff, its pronounced kre-at-un); and 2) color. If your piss is yellow but nothing else, then they know you took vitamin B12 to make your pee yellow. Creatin is a natural hormon your body produces, and clear pee is a "flag" that you drank alot of water for some reason. Thus you want yellow creatin filled piss.
The "detox" products you buy at headshops are not detoxifying you. All they do is provide a two hour window in which they mask the ton of water your peeing. It is basically creatin and B12 that has been flavored.
***** Day of test: Drink ALOT of water. Make sure you pee several times in the morning before taking the test!!! Many times they will schedule you for ~8 a.m. because your morning pee is has the highest concentration. Take the "mask" as directed (usually 45 minutes before). As someone earlier mentioned, take the sample from mid-stream!
GOOD LUCK! Let us know what happend!