What I did to prepare was made a power point slide show with a bunch of prompts for MMIs (you can find these in the interview section of sdn as well as various google sources).
Then, set up a webcam. Sit at your computer, pull up the slideshow, and record yourself. Give yourself a minute to think, then 5 minutes to answer. Watch yourself afterward (Yes, this is super painful) and learn from your mistakes - a lot of what I noticed was uncomfortable body language that I hadn't realized I was displaying, as well as pauses weren't nearly as long and awkward as I thought they were in my head. MMIs are testing your ability to think on your feet, so just being able to stay calm while reasoning out the question is a big part of it.