Synthetic urine, or urine that you know 100% is clean. This can be pulled off unless they want to watch you pee into the cup.
You need to have the temp. right, or the sample will fail on the spot. That is the 1st thing they check. Have it strapped to your junk for a few hours so it gets to body temp.
The body flushing can sometimes work. The problem is the flush also dilutes the normal body salts that should be in urine. The test can show no THC but that the normal body chemistry is diluted. That is an inconclusive test and most places consider it a failed test as that doesn't happen naturally.