Over the years I've interviewed for college, grad school, professional jobs, med school and residency, so here's my 2 cents:
Mumpu's last points are really important, particularly in competitive programs. You want to give them the impression that you've thought about why you're there (in that specialty at that program). I like to think of it in terms of "brand management," where the "brand" is you.
1. Be interestED (see Mumpu's points). HAVE QUESTIONS about them, the program, even if you already know the answer.
2. Be interestING: have traveled, have done research, have done something interesting besides school that you can talk about with passion and be remembered.
3. Be thinking ahead to your thank-you notes: try to jot something down that you talked about in each interview that was likely unique to that conversation that you can maybe reference in the thank-you, again, so you can be remembered. I like to think of the thank-you note as "putting your name in front of them and associating it with something positive and memorable (that is, the unique encounter that you'll jot down and reference in the not."
Kim Cox's points are also crucial; I couldn't believe some of the appearances I saw on the trail. Poorly fitting suits, old beat-up shoes, white socks. You want to stand out for looking professional, not for being the one guy with the purple zoot suit. With regard to her third point, recognize that some of these "hot topics" might be very controversial (particularly the one she mentioned), so keep it professional.
Finally, and everyone says this, but it's usually true: you're also there to interview them. Ask questions and pay attention to how it feels to be there; gather evidence about the culture to see if it fits with your own (this will help with point #1 above).