Quote Originally Posted by Burnt Toast
Any fluid consumption days before the test is useless. It may give your kidneys a workout, but the rate in which THC metabolites are filtered in from the bloodstream by the kidneys remains unaffected.

Therefore fluid-loading is only helpful on the morning of the U/A. Because the actions of fluid consumption are only temporary, once all of the fluids taken in had been urinated out, a 'dirty' donor will resume testing postive for THC.

For proper ways on diluting, read N2's Dilution sticky thread. Drinking more fluids than whats required in the sticky will increase the chances of the sample being ruled "too-diluted".
Drinking 64oz (recommended amount) of water increases your metabolism by 10%.