I did this and passed a drug test, I've smoked daily (missed about 4 days) for a year and a half, I weigh 145 though.
Day one: take 500mg niacin in the morning. Drank a gallon of cranberry juice, as well as at least a gallon of water.
Day two: repeat.
Day three, took no niacin due to fears that they would see a large amount of niacin in my urine and they would think I was trying to fake it, so I just drank as much water as I could stand.
Day four, drink plenty of fluid, made sure I was pissing clear, took the test and blazed a joint on my way home to hit my favorite bong

I got the job.