I take Niacin tablets, found in the vitamin section of any drug store. I'm a nurse so i get tested on a regular basis, every month like clockwork. I take niacin 2 days before my test, 4-5 tabs each day for 2 days, 1 every 45 minutes/hour. Along with huge amounts of water with every pill (you'll def. need it while taking Niacin). Atleast 20 oz of water every hour. Niacin sucks big fat hairy nasty sweaty monkey nuts, but it works. I smoke daily and have never failed a urine test ever since I started doing the niacin trick (few years so far). If you are not familiar with the Niacin affect... you will turn red (im white and its like having the WORST sunburn ever, even hurts to move because it feels like my skin is about to rip open lol) and you will sweat buckets, but it completely flushes your system. Its like every cell in your body flushes everything. Some people don't get it that bad, but I've seriously considered quiting smoking because I hate taking that stuff so much.
My sister drinks Apple Cider Vinegar and she swears by it. I have not tried it so I can't say for sure or how much to drink, but its much easier to drink than regular vinegar.
And some people do the gelatin trick. Go to any grocery store and by the straight gelatin (for making preserves and such). No flavor, color, etc. and mix it in with a full glass of water an hour or so before you take ur test and down it along with a load of water. Supposedly it creates a coating around your stomach and you will piss clean. Again, I don't know if this works. Never tried it. I just stick to what I know, and Niacin works everytime without fail. Hope this helps, sorry for the book