Reducing THC Metabolites in Urine
Things known to decrease the likelihood of a positive test.
Myths, Conjecture and Facts (for example, herbs like Goldenseal are useless against THC tests)

Fiber: A high fiber diet will help by redirecting fat soluble metabolites to the colon rather than bladder. "THC is eliminated primarily in the stool via bile acids. Both EMIT and RIA detect a secondary metabolite which is reabsorbed from the intestines. Thus a person with a high fiber diet will excrete a majority of THC [metabolites] in the stool" (anon1). A fiber-based laxitive will also help by binding bile-acids. Use caution. Fiber laxitives can alter one's bowel schedule and lead to dependancy. Pectin is a form of fiber.

Psyllium Husk: Psyllium Husk is an excellent, inexpensive source of soluble dietary fiber. It's available at natural food stores in bulk for around $6.25 a pound. A pound of psyllium husk will last for at least a month. The usual dose is several tablespoons a day in a large glass of juice or water. The commercial laxative product "Metamucil" is mostly psyllium husk. Psyllium's laxative effects are due to adding bulk and fiber to the diet. Ground or powdered form is much better than whole psyllium seed and preferable to whole psyllium husks.

Pectin: Certo, Sure-Jell, and MCP are brand names for fruit pectin used for canning and making jams and jellies. They are usually available in the baking supplies section of grocery stores. Pectin is a form of water-soluble fiber and is quite effective at removing bile-bound products (like THC-metabolites) from the intestinal tract and carrying them out of the body in feces rather than allowing them to be reabsorbed. Many older urine test FAQ's on drug tests mistakenly list pectin as a diuretic or contain pointless conjectures about it.

Note: "MCP" appears to be a modified citrus pectin product that is "more easily absorbed by the digestive tract" since the idea here is for THC metabolites not to be reabsorbed, MCP/Modified Citrus Pectin may be significantly less suitable for negating THC urine tests.

Dilution/Diuretics: Various methods to dilute the urine by drinking a lot of water and/or the use of diruetic herbs or medications are discussed extensively on the web. Dilution strategies appear to have significant merit, but are detectable when done to excess and various methods have been proposed to make diluted urine appear more normal to testing labs.

Emptying the Bladder and Producing Fresh Urine: Emptying the bladder completely and allowing it to refill shortly before testing seems useful. This prevents THC metabolites from accumulating over time as urine is concentrated by water reabsorbtion in the kidneys and perhaps the bladder. More information is needed to confirm this conjecture but it appears to be a sound and easily used tactic.

THC Detection Times in Occasional Smoker
Cannabis Urine Testing
by Texture   (at erowid.org)

Study Summary:
The study showed that in an occasional marijuana smoker that detection times for THC are approximately 24 hours, within 48 hours of ingestion the subject was testing clean(-) Detection times were determined to be so short that it would be possible to ingest THC on a friday and be clean by monday, thereby negating any test given after 48 hours of initial ingestion.

*1 The test was an immunochromographic urinalysis test configured to detect THC at levels of 50ng/ml
* the test used is a two panel test including THC detection at 50ng/ml and METH(amphetamine) at 500ng/ml
** Though we believe the results of this study to be reliable and verifyable - It must be noted that this is not a true controlled scientific experiment, it is merely provided as the results of testing for THC under real-world circumstances using a readily attainable ICA. This study should be confirmed by a controlled, multiple subject experiment before being purported as a reliable assessment of THC detection times. http://www.erowid.org/experiences/images/reports/exp-37341-urinetest1.jpg
Note: These are the actual tests used in the study. The image has been edited due to the unclear nature of the photograph to approximate the actual opacity of the test lines and indicate which tests indicate which tests are negative and which are positive.