Military:
Military piss test are usually random. They use a variety of ways to determine who gets to pee but the most common is using digits of one's social security number. If you partied a bit too much over the weekend and are a little worried about your results, here's what you do.
Since the test is conducted under the close supervision of a Staff NCO there is no room to use someone else's urine. The Sergeant will usually be watching you. Yes, watching it.
If you had the balls you could probably try one of those fake wieners that you can order in the back of High Times but I have my doubts about them.
The sure-fire way to guarantee you stay in clean piss is to drop a pubic hair into the mix while you're going. Drop a pube, you say? Yes, I say, although you might grab one that was still attached the alternative is popping on the test and getting a discharge that can haunt you for a long time. Although Walt Disney will hire anyone with a derogatory discharge, you don't want to be Goofy for the rest of your life.
The reason: The urine will be considered contaminated and untestable. The docs will move on to another jar and ignore yours. They expect a few of them to be untestable and you slide yourself nicely into their acceptable losses.
In a military setting, where random piss test occur on a regular basis they will disregard the contaminated pee and move on. In a one-on-one setting (ie. you got to go and do it for a job or probation) they will probably want you to come back and re-test.
All you've done is bought yourself some time. But it might be enough to munch Niacin and drink enough cranberry juice to drown a shark so you're sitting pretty next time.
Of course you could always just stop smoking pot, you big stoner.