I just had two MMI's and they were really not that bad. I think the best way to prepare is to 1) practice as you would for a traditional interview, since some stations may have traditional questions (why medicine? why this school?), 2) understand common ethical frameworks like utilitarianism, virtue ethics, etc., and 3) practice scenarios with friends:
http://zeroratio.blogspot.com/ or
http://www.medicine.usask.ca/pt/admission/Practice_MMI_QuestionsUofS.pdf
As for the interview itself, my best advice is to spend the 2 minutes coming up with 1) a couple of points you want to make,
and ideally a mnemonic so you remember them all once you're in there, and 2) a really strong opening sentence/thesis. If you start strong, you can kind of let the momentum carry you forward. And if you blank or get off track, you can always return to your thesis statement. At some schools, they will give you the prompt again inside the room, and re-reading it might help you out.
If I finished talking early, none of my 20 interviewers ever stared at me blankly for the last few minutes; they always asked follow up questions. If they are staring at you blankly, ask, "Do you have any questions for me/about what I said?" or, if you're grasping for more to say, bring up a common objection to your viewpoint and respond to it.
EDIT: Also, usually the prompts are very short. Some are just 1-2 sentences, so you can read them over and over. Even for the longer ones, 2 minutes is plenty of time to read them over 3-4 times and still have time to think about what you want to say.