I love this comment from Rebecca, on the topic of how to not be nervous in interviews:
Remember that all employers/bosses basically want to know the same three things:
1. Can you do what we want you to do?
2. Will you actually show up when you say you will, and do what you say you're gonna do?
3. Will you be a pain in my butt?
You and the interviewer have to work together to answer #1, because they don't know exactly what you can do, and you don't know exactly what they want.
Meanwhile, as long as your interviewer is reasonably sane, #2 and #3 will be answered simply by you showing up on time and being polite, friendly, and professional. (You would be stunned by how many people really don't do this.)
So really, as long as you behave professionally, the only thing you have to do in the interview is collaborate with the interviewer to figure out whether you're right for the job.
(It's true that the interviewer/company might not be reasonably sane -- but since you can't control or prepare for that, there's no sense worrying about it. Only worry about the things you can control.)