Drinking massive amounts of liquid before the test is another bad idea - labs easily detect dilute urine and reject the sample. Best case is you have to retest (sometimes under closer observation).
Plus their is such a thing as Water intoxification aka Water Poisoning that can be potentially fatal. So if a shitload of water flooded thru your system can kill you imagine what a similar amount of say cranberry juice might do to you.
http://en.wikipedia.org/wiki/Water_intoxication
Water intoxication, also known as water poisoning, is a potentially fatal disturbance in brain functions that results when the normal balance of electrolytes in the body is pushed outside of safe limits (e.g., hyponatremia) by overhydration, i.e., over-consumption of water.