I had a friend that bought this kit off of some website that was guaranteed to work, and apparently it did, because she's never failed a drug test. Sorry, but I forget what it's called.
On the other hand, I had another friend that tried to cheat by bringing in a sample of clean urine he bought off the internet as well. Snuck it in in a condom taped to his leg and the second it hit the cup, the temperature sensor said it was 5 degrees too cold. Needless to say that didn't work, and he didn't get the job.