I have several tips.
1. Be prepared to talk about "why medicine."
2. Be at least somewhat educated about the dr.'s specialty, what it involves, procedures, etc.
3. Be prepared to demonstrate that your a capable and driven individual, preferably one who is dedicated to healthcare for the "long haul."
4. Also, consider the "why DO" question. The importance of this will vary depending on the Dr. in question. The one I am with never used OMM, and was a surgeon, so this question was of less importance.
I had no idea what to expect when I first went to the meeting. I figured he would probably have more questions to ask then me. It turned out he was a former trauma surgeon in the armed forces, and I had a great time talking to him about his experiences. I also talked about some of the issues in healthcare that the US is experiencing right now - I have first-hand experience with it from my EMS career, and it was a good common ground to chat about.
The other Q's about lifestyle and such are good too.
Just go, talk, and be yourself!