since i'm sick of seeing people asking this question about urine testing, i'm copying this from a previous post i made awhile ago... if a mod would make this sticky it would probably save a lot of repeated posts...
when testing UAs they detect a metabolite which is basically what your body produces while a substance is travelling around your bloodstream. the minimum amount to be considered a positive test is when it is 50+ parts per million. diluting your piss means that in the sample there will be more water and less metabolites. if a test however is too diluted ( i.e. all water no other natural waste material that your kidneys filtered out from your bloodstream ) most will count it as a positive, or at the very least make you retake the test.
in order to get around this, instead of just drinking all water, drink about 33% water and 66% gatoraide. don't take any b vitamins... they don't do shit for your urine gravity except make your piss turn a bright neon yellow... when they see that they will know you were trying to get around it. the day before your test eat a light breakfast and an early lunch, and drink a large glass of water. the next day, try not to eat anything, but if you are really hungry just eat a little something at least 3 hours before your test. about four hours before your test drink like three-four gatoraides (16 oz bottles) and one-two glasses (16 oz = one glass) of water.
when you are taking the test, first piss in the toilet for a couple of seconds, fill the cup, then finish pissing in the toilet. most of the waste material is in the very first stream of piss, and the last little bit. if by chance you actually did dilute the piss too much, just tell them that you run/play sports a lot and easily get dehydrated... drinking that much fluid is normal for you.
again, let me reiterate; THERE IS NO PILL/DRINK/CHANT/ADDITIVE THAT WILL MAGICALLY MAKE YOU PASS A DRUG TEST. ANYTHING ELSE BESIDES LIQUIDS LIKE WATER AND GATORAIDE ARE A WASTE OF YOUR MONEY.
hope this helps.