If you are auditioning to play a certain part in a written script, get as much of that script as you can to read before the audition. It will give you hints on what to wear, as well as how to play the role. Do not be afraid of being too "on the nose"--subtlety will get you nowhere. If you are auditioning to be a hooker, get the fishnets and the see-through heels with a skirt up to there. Surfer--avoid a wetsuit, but wear board shorts, rubber thongs and some kind of surfer jewelry. Even if you know the "stereotypical" clothes are not the cutting edge of fashion, you still want to bat right down the center when you are trying to get a certain role.
While hookers and surfers are announced by what they wear, what about auditioning for a young girl in her mid-twenties with a bookish nature but who loves the hero of the film? Nothing is stereotypical about this, so the best advice is to look the best you can and as pretty as possible. Wear a wispy dress with a hoodie and worn cowboy boots. Show some depth and personality. Likewise, if you are auditioning for the twenty-something hero, look gorgeous. If you happen to look gorgeous with rumpled hair and a two-day beard, go with it. And if you are over 40 and auditioning for roles like teachers or professional people, dress very conservatively and wear your hair in a style that looks finished and clean. Never underestimate how much a pair of glasses will make you look more like a doctor or lawyer. Even if the glasses have no prescription.
This is where they want you to be you. But anyone who has watched anything with Flava Flav in it knows that they are looking for an out-there version of you. So if you are the guy who wears wacky T-shirts, wear the weirdest one you have. If you have a little bi-curious in your personality, let that show through. Guys in eyeliner and girls in a tuxedo stand out. If you are nature girl or guy and vegan, look the part and push the hippie aspect a little. Again, go for the stereotype of what your type is.