Um, let's see, my recs were from my previous research advisors (as far back as high school), and I did have a couple of (clinical) professors that I got to know when I was working in the ED over the summer, and a research professor at Maryland whom I worked with over the summer.
I think the most important aspects are to get to know the MD/PhD director well, and get his or her opinion on how likely or easy it would be to enter the program after first year. Show your interest, and I would also suggest trying to figure out what you're interested in early, so that when you DO interview, you can say, I want to do this project with this advisor in this amount of time. In my case, I think that showing the committee sincere interest in the program as well as having a plan and goals to achieve benefited my overall application.