I don't think anyone should have to be told this, but I benefited from some rather obvious advice, so let's be safe and cover it. Wear nice clothes to an interview. Even if you know for a fact that everyone at your prospective employers wears swim trunks and torn corporate freebie t-shirts, dress nicely. Unless you absolutely know how your interviewers will be dressed that day, Wear a suit. Even hypercasual startups with DDR rooms won't criticize you for it, and you never know when you'll accidentally interview for IBM.
If you can't afford a real suit, go to a thrift store. In fact, do that anyway, suits are expensive. If you really can't afford it, there are charities specifically set up to loan clothes to you for interviews and your first week (at which point you can go to thrift store and buy your own).