All I can say is relax...and that's actually what each my interviewers said to me. If it's a good program, they'll try to make you feel comfortable, (at least before the grilling

. I interviewes at CU and ASHS, so I can only speak in reguards to their programs.
I would first suggest going to the physicianassociate.com website. They have a few forums, one which lists all of the different programs. Here you can go under a program and post a request for an interview experience. There may even be one there now.
I would also check the schools website, and try to determine what they value in the education of a PA, and in incoming students. Check out all of their links. Then, go to the aapa.org website. It has tons of info. in reguards to the profession, it's mission, ethics, and current issues. There are also PA magazines online which have editorials reguarding current issues. This definately helped me to formulate answers to a few questions during the interviews.
There is also a student organization for PA students, I forget their title, but the AAPA.org site has a link for them. They have some info., ans a number of links to sites which give advice about interviewing. Also, the studentdoctor.net website has links to various site which also give interview pointers.
All in all, with all of the preperation I had, I came into the interview feeling fairly confident which helped to relax me. The biggest factor which helped to sooth my nerves was a backup plan. Realizing that alot of competitive student don't get in until their second try forced me to develop an alternative to school. After I developed this I went into the interviews ithout the pressure of "I have to get in".
I hope this helps, and from what I've been told, schools usually only offer interviews to students they feel are qualified. So you should be psyched to be in the minority :wink: .
-Good luck!