What really helped me was the night before my interviews I went over SDN feedback and found a few of the most common questions, and I'd think of what I'd like to say (don't script yourself), and I'd try to say it out loud as though I was just asked the question - might look a little silly, but helps. Be sure to know the schools mission statement - usually always education, community service, and research. Also like others said, go to their website, and see if they had anything big happen recently, like when I interviewed Pitt. they recently opened a big Special Needs dentistry clinic. Maybe all in all an hour for everything.
And yeah, preparing for the interview is the stressful part, once you're there in the hot seat it's really not that bad, just let it flow out naturally. Good luck!
