In the event that you've known about this upcoming screening, or the possibility thereof, my first piece of advice and the first step to getting clean urine would be to stop smoking marijuana. If you've known about this screening, or that a screen was imminent, with some important aspect of your life hanging in the balance, and you've continued to smoke throughout the period of time where this screening was possible, you may have an addiction.
Other than that, I've seen the rest of the advice these guys have posted work time and time again for my friends. One thing about detox kits performing or failing is that from box to box each detox kit is the same, person to person everyone has a great deal of variation. Factors, of course, such as weight, body composition, and frequency of marijuana use can all have a hand in the success or failure of a detox kit. Like Rogaine and Wartol, expect the worst, hope for the best.