also make sure you don't give them your first piss of the day, give that one to the bathroom.
Also it depends on how much you smoke, how long ago (if you smoke a gram a day, then going dry for a couple of weeks will help), and what type of shape you are in also.
Because it is stored in your fat, and when you get out for a run/walk/jog and start to burn that fat off, it is then getting rid of the stored up THC in your system.
Drinking cranberry juice and such helps, but if you over do it, your piss might be to watered down, and they reject it on that alone.