If I were in a situation where I had to pass a THC test, the first thing I would do is put down the bong. Then I'd find my swim trunks and running shoes.
Like it was mentioned before, THC is fat soluble. So, start exercising and try to burn off some of the fat holding the drug. To help it along, eat a low fat diet and watch your calorie intake. Don't starve yourself, but also don't eat a loaf of bread a day. Run, swim, exercise and drink plenty of fruit juices and water.
Another big thing is knowing what sort of test is being used. If it's one of those on-site thingies where they use a test strip of paper, you'll probably fool it by drinking a shit load of water and coffee: you'll have so much piss in you that the THC will be very dilute (Tip: dose up on vitamin B to color your pee yellow cause sometimes they'll turn down a specimen if it's clear).
If they're sending a sample away to a lab, they're going to be using mass spectrometry or chromatography. These machines can detect molecules at the ppm range... you won't fool them.
There's a lot of wacky pseudoscience floating around that won't help. There's only one way to get shit out of your body quick: your own body. It's been practicing it for millions of years through evolution.
My $0.02
Passing a drug test can be quite easy. I've passed two lab tests the past year, the first with a 3 day break and the second with a 5 day smoking break. In order to beat the test you have to understand what they are testing for. Obviously they test for THC and THC metabolites. The goal is to dilute your urine enough so that your THC metabolites are below the cutoff levels of 50ng/mL without creating a red flag in your sample. Drinking tons of water will make your piss crystal clear (red flag), and your creatinine levels will be way low (creatinine is the byproduct of creatine, found in meat and what some bodybuilders take).
How to beat it then?
Color: Riboflavin (Vitamin B2) supplements. Riboflavin is orange and consuming it in excess will turn your piss neon yellow. Don't worry about overdosing, B-vitamins are water-soluble and are simply excreted if not needed (hence the neon piss).
Creatine: Creatine supplements, obviously. If you look at ANY "detox" drink, two ingredients are ALWAYS present: riboflavin & creatine.
"Detox" drinks do not detoxify anything, your liver does that.
Factors that will raise your chances of beating it:
1) Low fat levels: THC is fat-soluble and those without much fat will store less
2) Exercise: Exercise will raise your metabolism and will help burn off those metabolites. Avoid exercise 1-2 days before your test to keep yourself hydrated.
2) Drinking LOTS of water. Toss in a Gatorade or two to prevent cramps from low sodium and/or potassium levels. Or eat a banana.
The morning of my test I took my prescription Adderall (15mg XR) so I knew I would get a phone call when my results came back and I wouldn't be left in limbo wondering if I passed. Well, the test didn't even show up positive for amphetamines!!
Hope this helps all