The easiest way to pass a hair follicle test is to refrain from drug use entirely. Alternatively, find an appropriate hair treatment such as a white vinegar mix to wash the chemical residue out of your hair. These treatments, however, can leave obvious bleaching and tip off employers to potential problems.
Hair drug testing is utilized because it detects drug use that has happened up to months or years ago. These tests cannot detect drug use that only happened a few days or a week ago, due to the average rate of hair growth, so some employers use urine drug tests in combination with hair tests to be absolutely sure they catch potential drug users. Simply cutting off your hair runs the risk of an employer noticing it, if you have been working for a while and cut your hair when drug tests were announced.
Even if you do not consume any drugs, legal or illegal, some medications and chemical treatments cause false positives. Nicotine shows up even if only temporarily in the presence of smokers, and cold or flu medications have compounds that cause false positives for methamphetamine-type drugs. It is possible to dispute drug tests if you have evidence of false positives such as these, but some employers do not accept disputes and terminate or deny employees who show any positives at all.