Option A(one I recommend as it has served me well throughout multiple drug tests over the years): Ask a friend who doesn't smoke to piss in a water bottle for you, get a pair of gloves and one of those smaller contact lens solution bottles. Put on the gloves, rinse out the solution bottle and then pour some piss into it. Get a glove (optional) and a belt. Buy some hot-hands or other similar hand warmer from a gas station like 30 minutes before you have to take the test. Have the glove belted to your inner thigh so it isn't obvious if they make you keep the door open. Handwarmer should get it warm enough (if you do like I used to and activate it in the gas station bathroom while you're belting the piss to your thigh so it has time to warm up) that they're not like "wow your piss is really cold". The only downside of this method is, if you show up and there's unexpectedly a line, you can potentially give yourself a nice burn from the hot-hands like I once did in high-school when I showed up to take the drug test for a local themepark and there were like 30 people in the room also waiting to piss in a cup while the one lab corp technician stood by the bathroom door.

Option B (I've only had this method work once and I am an extremely skinny pothead, so you'll probably have better results but it isn't my preferred method since the chances are slim for someone whose minimal fat cells are probably pure THC at this point): Exercise, drink cranberry juice, and make sure you've pissed like at least twice the day of. Then it's just a matter of diluting your piss with heavy water intake. Vitamin B12 and whatnot as others have said helps.