I have sat on several interview panels where I am not involved in making the final decision but decide who moves on to second interviews and give my opinion to the person in charge.
First impressions are always really important, meaning how you dress and how put together you are. Based on some studies I've seen, better looking people tend to get hired over the less put together, so make sure you look your best.
In most cases, there is no such thing as overdressed for an interview, so unless you have specific instructions to the contrary, NEVER wear jeans to an interview.
I am also commonly surprised at how often candidates do not make eye contact, and how often their answers are unrelated to the questions we ask. It's okay to be nervous, it's okay to let the interviewer know you are nervous, but it's not okay to be incompetent, so take your time, BREATHE and ask them to repeat the question if you need to.
One time we I asked someone why we should hire her and she actually said, "um... I don't know". She wasn't hired.
Now, beyond looking nice and answering the questions well, there is a lot that goes on beyond that. There are plenty of questions employers don't ask you, but they are asking themselves.
These are some of the questions behind the questions that employers ask in interviews.