To add to that. If your urine is coming out clear they may have a visual suspicion of being diluted. Drinking gatoraide and poweraide will help with raising creatinine levels (or you could take a creatine suppliment). As everyone said, water days before hand wont make a difference and doesn't "flush" other than time.

Be careful with a negative dilute, some companies consider that a failure. To get to a nice yellow color, take some Vitamin B2 the morning of about an hour or two before. Void a few times (2-3) and by then you should start coming out vitamin yellow. That will pass the visual and give no concerns for worry. Add the creatine in and you will make sure that your specific gravity is correct as well and lessen chances for having a negative dilute.

To explain negative dilute, thats when the test shows negative results (pass) however, there is an abnormality in the specific gravity (from water loading) that shows you attempted to fool the test. Clear piss will also do that. I think your levels have to be above 20ng/ml to not show a negative dilute. Read the sticky on dilution, follow it, you will probably be money in the bank. Again, your time, weight, metabolism, etc may let you just pass naturally without a dilute. Or get away with a bottle or two of gatorade in the morning of your test.