In the end, there are only two ways that work: lowering the amount of THC in marijuana and speeding up the metabolism. If you drink enough water, your THC levels won't be too high on a drug test. People whose test results are close to positive or negative may have a better chance of getting a positive result if they are dehydrated. There's no foolproof way to speed up your metabolism.
Even though exercise may help the body get rid of THC, doing it too close to a test could make you test positive. The most important thing is how long it has been since the last time the person was exposed.