I've had 3 with another one scheduled so far and I love them!
There are three things that I've found useful for preparation.
1) Read the UW page on Bioethics. No SERIOUSLY. READ IT. Every MMI I've had has drawn scenarios directly from it. Additionaly, if you can think of other murky scenarios not on the site, read about them, too. (Eg When do I legally have to report abuse? What about when X.? etc.)
2) Start talking to randos more. Guys next to you the train, girl in front of you in class, dude sitting next to you on the plane to your interview... ANYONE. The point is to get comfortable in situations where you otherwise might not be.
3) We all have "that friend." You know, the extremely argumentative one with deep seated opinions? Talk to him/her/them to get comfortable stating a position and then maintaining it. They'll poke holes in your argument. This is VERY good. MMI raters will do the same thing. They also might try to get under your skin. Practice keeping your cool. You can talk about scenarios relevant to medical ethicss, but also pretty much anything else under the sun... religion, 2016 candidates, the wage gap, GMOs... anything you can have discourse about, do it!
Good luck!