unless you're a 500 lb. monster who smokes oz.'s of weed per day, you will be clean in 13 days almost guaranteed. if you're still worried, there's 2 things you can do. one: get someone else's piss and put it in the cup. make sure the piss isn't more than about 12 hrs. old though, and make damn sure its clean. they also may check the temp., so heat it up somehow (handwarmers are good).
two: if they don't test for creatanine levels, you can start chugging water 3-8 hours before your piss test. you should be pissing about every 15 minutes, and your piss should be clear as hell. to counteract this, pop a vitamin b complex pill an hour or 2 before the test. now, if they test for creatanine levels, your levels will be too low and they'll know you watered your shit down.