Christ, I post this every time I see the word piss next to test.
My only recommendation for beating a piss test [if you must smoke weed] is ‘herbal cleansing’ Carbo Clean 32 [available in Grape and Fruit Punch flavors], about $60 at GNC, though you may be able to find it somewhere else.
I'd drink it t-minus 2 1/2 - 3 hours. Piss as much as you can. "The test is supposedly active from 1 or 2 to 5 or 6 hours." The more times you piss the better it is. You won't have to go too much, so after you drink it, piss every ten minutes, force yourself [not joking]. The best method to beat your piss test is to not smoke [or smoke less]. Another thing to look into is JWH-018.
Also, get a kit from walmart [or any store] to test yourself. TRY to piss dirty for this test. Fill the first half of what you need in the cup with the beginning of your urine stream, and the rest from the last drops of your urine stream. This is ideally the first piss of your day. If you go clean on this one, save your clearing kit. Eat a big fatty meal, don’t exercise, and only drink a little extra water. NOT TOO MUCH! This will fuck up your creatine and specific gravity if you get selected for lab testing [This, the previous part, is ONLY if your attempted dirty test is clean]. If you're dirty... use your clearing kit and buy another clearing kit, and if you want some peace of mind, test yourself with a walmart test before the real test again to make sure you're clean [always are]. Good luck man, so many people give false information over piss tests, they say things like 'niacin, water, cranberry juice...' I did the research, I lived it, and I experienced Carbo Clean 32 beating a PROBATION LAB TEST. This test does GC/MS [toughest piss test ever] and adulterant tests [also tough to beat, detects if you used a clearing kit], including specific gravity and creatinine. <-- and specific gravity and creatinine levels will be FUCKED from water-loading.
Also some tips to help yourself stay clean [no offense here if you live a PETA lifestyle] Eat a shit ton, and eat a lot of red meat. This will boost your creatinine levels naturally. Exercise so you don't get fat. The more you eat the more you shit, and where I'm going with this, is all of the THC absorbed into your system, 80% of it leaves your body through your shit, and 20% of it leaves through your piss. Each time you take 80% away of the THC, the less likely you are to piss dirty. Dark liquor [best], though any will work... (avoid beer a few days before and after smoke sessions) [enough to get good and buzzed (I prefer enough to get FUCKED UP)] 2 hours prior to smoking will help to prevent "Note I didn't say entirely prevent" THC from bonding to your fat cells, causing an equal amount of high from smoking, and much much quicker 'spent' or 'metabolized' THC will be eliminated <-- which is what they detect in a test, active THC will not be detected.
I'm sorry if this looks like a speech, but I want you to definitely understand a VERY brief overview of my countless hours of piss-test-research findings.