Cool, calm, collected. Know your stuff on current medical topics (healtcare issues). More important than what you say though, perhaps, is how you say it. Don't appear razzeled, no matter the question.
As for how best to prepare, everyone has their own individual ways on how to prepare for the current events/medical topics, but I'm pretty sure everyone would suggest the same thing for info about the school: SDN interview feedback and the school's website.
-do mock interviews!!
-watch national/international news (shows like dateline are great especially when they examine healthcare)
-arrive the night before and relax
-know their program/curriculum and be able to discuss it (good and bad aspects)
-know why you want to go to their school
-pause and spend a few seconds to prepare each answer
-answer the question that is asked!!!
-be succint (avoid rambling on a tangent)
-DON'T contradict yourself, but DO analyze both sides of an ethical argument
-drink the water they give you (a good time to pause and take 5 seconds to prepare your answer)
-look good!! (skirt/pants/blue shirt/white shirt/colored tie/patterened tie/hair up/hair down/pinstripe/etc/etc/etc... none of it really matters - just look formal and professional and confident)
What everyone is saying here is true, but I think the #1 thing is for whatever you're saying to ring true. You just have to be honest. If your interviewer thinks you're BSing, that will hurt you regardless of who is interviewing you.