Are you on a weight loss streak?
THC & THC-COOH are stored in your fat. If you were a heavy user, and gaining weight during that time, THC is stored in your fat. If you start losing weight, as you lose the weight the THC goes back into your blood stream as your fat is metabolised to energy.
I've heard that you want to be inactive, or on a weight gaining trajectory, to not have fat stored THC going back into your system.
Only the urine THC drug test picks up your THC-COOH (metabolized THC from fat stores)
This site has what seems like some good tips.
Water
There is no reason to start drinking excessive amounts of water weeks or days before your test. It’s a waste of time as it only temporarily dilutes your urine. Your best bet is to drink 2-3 liters throughout the day before your test, and then around 1-2 liters several hours before your test. Don’t drink too much water though, as it can be dangerous. Start with 2 liters max and be sure to know your limits.
After drinking this much water, your pee will be colorless and have low levels of creatinine. These are both red flags for any testing lab to indicate diluted urine, so there are some additional steps you need to take.
Vitamin B
You’ll need to make your pee yellow, and the best way to do this is by using vitamin B. Vitamin B-2 or B-12 seem to be the most effective for this purpose, so stop by your local drug store and take 50-100mg several hours before your test.
Creatinine
Creatinine is a chemical waste product produced by muscle metabolism. Creatinine is naturally occurring in the human body and is one way labs determine if your urine is diluted. It has a half-life of three hours, so no reason to start taking it more than a day or so before your test.
You can buy creatine supplements from almost any health food store. Creatine then gets broken down into creatinine by your body and is flushed out in your urine. You should take an above-average dose several hours before your test.