I've never done this, but my hubby has perfected it.

Get a small jet dry bottle, and an A/C or meat thermometer. Clean the bottle out extremely well. Poke the thermometer through the top of the bottle, beside the nozzel. Then seal up with tape, silicon, or a glue gun. He also attaches a string with a clip on it to the bottle. Get someone to piss in it. On your way to the doc's office to take the test, clip the bottle in front of your vents, and turn the heat on high. You'll be able to easily see the temp too. Heat it about 10 or 15 degrees over what it should be, and clip it inside your boxers next to your "stuff". It helps if you kinda keeps your legs together to. My hubby and I sat in the waiting room for 30 minutes and he still passed. When you go to pee, simply pull it out of your zipper, pop the top, squeeze it, and it actually looks like you're pissing. It's helped him and alot of his friends out with job drug tests so many times

Good luck