PhDuke - I wrote an article on my blog about how to prepare for interviews. You can check it out here.
www.road2md.com/#blog
One thing I didn't cover is preparing for the MMI's. There are some decent resources out there.
http://medschooladmissions.com/medical-school-interviews/medical-school-multiple-mini-interview-mmi
Caveat: Information seems legit but I'm not sure who wrote these things.
http://dukemed.duke.edu/modules/ooa_applicant/index.php?id=9
http://multipleminiinterview.com/mmi-questions/
Caveat: Doesn't seem like actual physicians wrote these things but again, a decent place to get started.
In terms of practicing, there is nothing better than making it as realistic as possible, so I would try to get people with experience interviewing med school applicants. It helped me tremendously when I was in your shoes. Interviewing at a bunch of schools will help you get more comfortable, but you don't get feedback.
Good luck!