Drinking gallons of water alone will not help you. In fact if you drink too much you could get a "Dillute" and then they will probably re-test you. Exercize is important. Vitamin b12 to make your piss more yellow.

Too much water can lead to Water Intoxication. Water Intoxication & Hyponatremia

Not likely but it could happen.

Metabolizm is another factor. Some people metabolize the thc quicker. I think a week's time is too short if you have been smoking for 2 years straight. I'd say minimum 2.5 to 3 weeks. When/if I need to take a test i'll have to quit for a month since ive toked every day for years too.