Niacin is a myth. It does nothing to help pass a U/A. Also, taking considerably more niacin than the recommended 15 mg/day can have some serious health consequences.
Morons Overdose on Niacin While Trying to Beat a Drug Test | Wired Science | Wired.com

Medical News: Overdoing Niacin Can't Thwart Drug Abuse Tests and Is Risky - in Emergency Medicine, Emergency Medicine from MedPage Today

Drinking fluids is only helpful on the day of the U/A. Drinking a bunch of fluids days or weeks before the U/A is of no help. It may cause frequent urination, but it has no effect on the filteration rate of THCA from the bloodstream by the kidneys.
For proper tips on waterloading, read this thread:
http://boards.cannabis.com/drug-test...uidelines.html

To bring color back to the urine, the proper vitamin to use is B2 (Riboflavin), not B12.

Sweating doesnt help much because very little THC is excreted thru sweat. The rest is being excreted via the urine and bowels.