What I'm usually looking for when I'm doing interviews is how people carry themselves. Don't offer information you're not asked for, unless it directly helps while answering a question asked of you. You don't have to be honest, just convincing. Have all information you may need with you, paperwork and such, that you may be asked for. Also, ask questions about the job you're interviewing for. Sit up straight and make eye contact.
What kind of questions should you be asking as an interviewee? Besides "When do I get paid?"
It's kind of specific, but when I was interviewing (for architecture), I always made sure to ask each firm how they would help me with the IDP (Intern Development Plan). It's a mentoring program that all architects have to go through to be registered, and requires fulfillment of a variety of criteria covering different areas of the field. I'd check and see that they had put other interns through the program, that they covered all of the requisite areas of study, and that were encouraging about the whole thing.
In general, my attitude with interviews is just be be natural and honest. That's what worked for me with my current job, and I think it's a large part of why the workplace suits me so well. I talked about what I was naturally interested in, and it just happened to synch up great with what the firm did. I realize that it's a luxury that one can't always afford, though.