As previous people have stated, look for questions that have been asked at the particular school. Look here on SDN at the Dental School interview reviews for your school. I found literally hundreds of questions that have been asked going back more than ten years at my school. Unfortunately, depending on how structured your interview is, I ended up only getting one of the questions I prepared for (the rest of the interview was lax).
Dress well, be polite to the staff (don't be fake-you know when you are doing that).
If you can, scope out where you will meet for the interview the day before, or have someone (i.e. a current student) take you to your location, do this. Less stress=more success.
Have your own questions to ask, multiple of them actually. I stumped both of my interviewers with one particular question. I can't confirm or deny that this helped my chances, but I'm sure it is different when they can't answer a question, when they are more accustomed to that from the other perspective.
Thank your interviewers via email afterwards, if this wasn't a given.
Also, know the particulars about your school and why you want to attend there. Really key in on the out-reach programs (does your school offer a clinic where you put sealants on kids' teeth in your first semester? Then state why you are excited to get early clinical experience especially with kids) and any actually enticing things about your school. Do they give B+ to distinguish you from someone who only received a B? State how this could be useful for residency and why you like that system. Is there a particular aspect of a residency you like that they offer? State that you are either interested in that specialty or that it would provide a great learning environment to be a particularly competent GP. Moral of this long paragraph: have more responses to questions than you will be asked.
Good luck, it's a truly exciting time.