I used Certo brand fruit pectin and Niacin to clean out my ex for probation tests. It worked every time. You double the amount recommended on the bottle of Niacin and use 1 package of certo per 8 oz glass of water at least 2 hours before the test and another package 1 hour before the test.
Thank God we weren't scientists trying to figure out why or why not..

Also...the average person is supposed to drink six 8 oz glasses of water daily.
Hardly anyone does. If you did drink that much water and cut out the things that make you dehydrated like soda and caffeine it'd be easier to clean out quicker. Pot is a fat soluble drug so it stores in your liver.
If you're dedicated to the six 8 oz's of water put about 2 teaspoons of Apple Cider Vinegar in one of the glasses. It'll help. Make sure you mix it very well before drinking it.