I wouldn't rely on Caffeine to increase your metabolism, it's not a proven scientific fact. Having a fast metabolism is however a good thing for someone who smokes marijuana and gets drug tested. If you don't have a fast metabolism and want to increase it, then you need to exercise regularly. Eat small frequent meals as well as opposed to few large ones. Iced water is also known to increase metabolism by cooling off the body. The body then burns calories trying to warm itself back up. These things and time alone are the only way to rid the body of THC.