How do you cheat a marijuana urine test?

By picking a different line of work. It is absolutely permissible for your unit drug test sergeant to knock on your door at 3 AM and escort you to the nearest latrine for a test.

Here's a fun trick I've used myself - yes, I was a drug test sergeant. I went to the post drug test lab and picked up fifty specimen bottles. Before I went out and got in my car, I went to the company commander's office and, with the door open, we discussed the upcoming drug test in a loud, clear voice. And then I asked the company clerk to make sure I had tables, chairs and other things I needed for the drug test tomorrow. As you'd expect, within twenty minutes everyone in the company knew there was going to be a urine test tomorrow morning. Which meant everyone was working his/her tail off to flush the drugs from his/her system. Come the next morning we did the collection..but instead of testing all that urine, upon arriving at the lab we immediately dumped it down the sink. Three days later, after everyone thought they were in the clear and blazed up to celebrate, we announced that there was a "problem" with the last collection and the lab wanted us to test everyone in the company. We caught and kicked out 25 people that day.