There's only 3 ways I've ever seen work to pass piss tests, none of which is from trying to clean out or flush your system, it just doesn't work. The 3 that do are:
-Don't use your piss. Take a clean person's piss in with you discreetly. (Assuming they don't watch you piss.)
-Take in synthetic urine form a head shop. This shit works every time! (Again, assuming they don't watch you.)
-There's a nasty drink you can get form head shops that you chug down like an hour before your piss test and what it does is temporarily bind up all your toxins in your body so when you piss within a certain time frame no toxins will come out.
A few things that DO NOT work are, cleanses, teas, drinking Pectin, drinking tons of water, drinking bleach diluted in water, none of that shit works, don't even bother trying it.
Peace. =)