Don't look unprofessional
Maurie Backman: Of all the things not to do during a job interview, showing up unkempt and dirty is probably the worst offense I've ever witnessed. Back when I worked at a marketing company, we had a candidate arrive for an interview looking like he'd just gotten back from a multiweek wilderness trek and hadn't yet had time to stop at home and clean up. His hair was messy, his khakis were dirty, and rather than slip on a pair of business-looking shoes, he opted to show up wearing mismatched socks and Birkenstock sandals (an interesting fashion choice, to say the least).
Not only did this candidate make no effort to dress professionally, but it was obvious from the second he walked into the room that he'd neglected to shower for multiple days straight. Ironically enough, he seemed like an otherwise intelligent applicant, and he shared many good ideas during our interview. But I just couldn't get over his blatant disregard for proper attire and hygiene, and ultimately voted not to bring him back in for a followup, as did the rest of my team.
If there's one takeaway here, it's that it always pays to look professional when going on an interview. Even if the folks you're meeting with are all in jeans and flannels, you're expected to show up looking like you mean business.