I won't go into detail about job interviewing; that's better left to the experts like Rowan Manahan over at Fortify Your Oasis.
But I have two simple tips today that I promise will put you slightly more at ease in an interview setting.
1. You are not the only one in the hot seat.
Your brain is screaming, "Pick me! Pick me!" But you are evaluating them as much as they are evaluating you. You may need a job, but they need an employee (or intern, or consultant). It's great if they pick you, but you have to want to pick them, too.
2. It's a mutual fact-finding mission.
They will have a lot of questions prepared in order to determine if you'll be a good fit with their institution. Don't forget to have your own questions prepared so you can find out if they're a good fit for you.
You are not at the mercy of the interviewers, and it's not just their decision to make. Both the candidate and the interviewer are trying to find the best match to their needs.
Have confidence in your own ability to evaluate your evaluators, and remember that you are making a decision, too.