It'll really help to know what KIND of test you'll be taking. Is it going to be sent out to a lab or will it be an instant cup? Is this a pre-employment drug screen (DS) or something for probation/parole/DCF (dept of children and families)? Is it observed or just a "empty your pockets sir"?
If it's just a pre-employment screening or an Instant Cup test, then don't smoke from now til your test, drink lots of water, don't excercise, and take a good amount of B-complex vitamins each day. No worries.
If it's going to a lab, then you'll need a real flush ahead of time (several good ones are mentioned above). Again, stop smoking for the whopping 10 days, drink plenty of water the day of, take some B vitamin, and follow flush instructions to the T. That simple.
If it's for something like a DCF investigation, you'll need a synthetic urine for the most part. They know this, so alot of those are observed. No worries about probation/parole tests. They're observed urines but they tend to use the cheapo instant cups or cheapo labs. The DCF ones tend to use a technology called GC/MS (Gas Chromatograph/Mass Spectrophotometry). You simply WON'T be able to beat this technology with any flush. Same tech they use in forensic labs. No joke. Synthetic urine or clean urine will be the only reliable way. Hope this helps boss. And again....don't smoke for these 10 days. If you can't make it the ten days without shooting up an office or something, then you're tolerance is too high anyway and you could use the break. "I just saved a boatload on my ganja bill, by not smoking for a week!".
-Butters (RIU member and Licensed Toxicologist....no bullsh)