Heavy/Frequent smokers will build up THC overtime through the bloodstream, which some gets stored into fat cells (got the munches). It's the THC in the fat cells which can stay in your system for months on end. You don't even need to be fat. When those fat cells are burnt off through exercise/sweating, the user might experience mini-highs (most don't like). You need to burn this off, but timing can be a factor of a 6-12 month period at least.
Note also it is theoretically possible for a chronic cannabis user to fail a drug test long after the usual detection time due to exercise, dieting, or severe stress shortly before the test—and several anecdotal reports of this exist. However, there is currently no hard evidence that enough active THC would be released to get one "high" or cause "flashbacks".
This negative impact has been clearly shown in brain scans, in intelligence testing, and from self reports of youth who have quit using for at least several months later on.
Because daily use results in the filling up of fat cells with THC, those fat cells will release a great deal of THC back into the bloodstream, resupplying the brain with high levels of the drug every day after the user quits. Since levels remain high for at least a week, the user will experience fewer withdrawal symptoms after stopping than would occur if he had been using any water soluble drug. This explains why marijuana is considered to be non-addicting, when in fact it is a very addictive drug.
Coming back to mini-highs, not an actual high. The heavy user quitting over time might still experience extreme laziness, headaches/migraines, even worst case blackouts, due to these or just a weird dazed feeling. You should weed yourself off it slowly, exercise regularly, but not exercise around testing date to avoid any more release back into your system. It's possible, depending on how heavy user you are, you will still fail the test. If you are doing hard exercise to get them out of your system, stop as much physical activity as possible 2 days before the test or they will definitely show up in your sample.
Drinking water or using flushing agents DOES NOTHING to remove the metabolites from your system, the only thing drinking lots of water does is dilute your urine sample the day you are tested so they register under 50ppm, that's it, period - quite frankly it isn't all that great for your bladder to flood your system continously. Most tests will detect this and simply get you to retake the test again or fail you for diluted traces. Do drink days in advance, but never heavily around testing day.