I've been in my current job for a couple of years now, but before this, I spent a whole lot of time in interviews. One big tip that I picked up from many different interview panels is that how you look really has a big impact.
Now, I don't mean your actual looks. You can be tall or short, fat or skinny, brunette or blonde, and none of that will matter. But how you put yourself together definitely will.
Of course, this advice may not apply to every job interview, but in general, I think that everyone should wear a nice suit to an interview. A suit that fits well and looks professional. It doesn't have to be an expensive suit (though suits are not cheap). But you should look nice and well put together. Try to keep your pants from dragging on the ground, for example.
I have had a lot of friends go on interviews lately, and after seeing what they wore to the interviews, I had to prevent myself from physically cringing. It's not that they looked bad. In fact, most of them looked really good.
Perhaps a little too good.
Ladies, cleavage isn't appropriate for a job interview (unless you are applying at Hooters). While some interviewers may not be bothered by it, and some may even enjoy it, you're risking looking unprofessional. Same thing goes with a too-short skirt.
Guys, you can look trendy without looking too trendy. A friend of mine loves to wear spats. I think they're a lot of fun, and fine for the office, but perhaps not the best thing to wear to an interview. You don't know how others will take that. Some may think you're fun and interesting, but some may think you're a goofball and not ready to work.
The one thing I was always taught by various career counselors is that it's best to be a little conservative in your dress when you go to an interview. Stand out with your personality and your resume, not with your clothing. While they'll certainly remember you if you wear a pink plaid suit, wouldn't it be better if you were remembered as the interviewee who totally nailed the second question?