Even if you stay clean before the test, your hair can serve as a long-term timeline of drug use. I have no idea about body hair, but the hair on the top of your head can be used to test for drug use months before the test (depending on the length of your hair of course). Even shit like coke, that's untraceable in urine after no time, can be detected way after doing it. I'd say your best bet before going into a test knowing you've done drugs is to cut your hair shorter (the longer it is the longer the timeline) or claim (in the case of THC) that it was outside contamination- say you went to a concert and must have unknowingly absorbed some of that 'awful' smoke.