I've sat in on countless interviews very similar to the one you're facing, and for me, the things that set quality interviewees apart are honesty and preparation. Honesty comes through--it's not hard to spot someone who doesn't know what they're talking about and is attempting to cover for it. It's also easy to spot someone who isn't sincere in their answers. So be honest. If you have areas where you'd like to improve, say so. It also helps to be able to speak in an informed manner about the position you're trying to get. Speak in specific terms. Don't just say you'll do a good job, give specific examples and how you would handle them. It also helps to be able to come up with specific examples of what you've done in the past that would make you a good leader.
My advice is to confidently walk in the room and let the interview committee get to know you and your heart. Be confident knowing that if you answer the questions honestly, and sincerely and let them get to know you, you will get the job if you are what they are looking for. If the committee doesn't think you are a good fit, then you probably wouldn't enjoy the job anyway.
It is unfortunately all too normal to feel nervous and self-conscious during a job interview. I have found that the best way to act in such an interview is to be well informed about what the position will entail and to ask questions. If you can get the interviewer talking about the organization and the duties, it will take away some of that feeling of being under a spotlight and being given the third degree. It will also convey the impression that you value yourself and are choosey. If you follow this advice it could make you feel more relaxed and in that respect different from a lot of the other people being interviewed. Naturally you should have a good-looking resume to submit to the interviewer. This will give him or her something to look at and something to talk about. If possible, it is worthwhile to practice by having some friend interview you.