hey mate, this might be of more help to you if the above doesent appeal..

I know of plenty people who passed tests by drinking litres of water before (preferably at least a couple of hours if not more) but you must also take some creatin or creatine crystals..they are used by body builders to make muscles stronger. This is because the urine will have some natural creatine as excreted by the kidneys and if you drink all that water well they test for creatine and so you must replace or you will fail. Lastly, a good STRONG multi vitamin or preferably a 1 gram super strength vitamin B to colour your piss so that its not clear. It was a chemistry professor who had to think of that one to save his skin and it soon became a tried and trusted trick. This was 8 years ago but i imagine its the best bet.

ps i imagine coffees good whilst your at it.