I completely agree with the previous posts...Know your application well. Be prepared to answer questions about your application that may catch the adcoms eye (Like a low grade, low score on a section of the dat, heading up your own research project/publication, etc.)
Also, check back the interview feedback forum on sdn and look over the questions that were asked in the past.
Another good thing to do is go to the school's website and review that. See what makes that particular school unique and base some of your questions for the adcoms about that. Do they have a PBL style curriculum? what percentage of students pass their boards the first time around? Is it a struggle to get a chair/patients in clinics? What are the social dynamics of the previous classes? Are the students close knit and help eachother out, or is it more along the lines of you are on your own. Find out how helpful the profs are outside if the classroom if you run into a problem. Asking questions will show you are interested, as well as show that you are concerned with the education you will be getting, and not just concerned with getting in somewhere (even if that is the case)
But I think the most important thing is to be yourself. This is the most overstated advice you will get from anyone, but the most important. Be confident, but if you are nervous and have a nervous tic or sweaty hands, let the adcoms know and laugh it off with them. Keep in mind that adcoms are people too, and they aren't there to scare the hell outta you. They just want to get to know you and know that you are a social human being.
This pretty much sums it up what I did. I only had 4 interviews, but got accepted to all 4 schools, so i am thinking my interviews went pretty well. I also only reviewed the schools website/questions on sdn the day before the interview, and for only about an hour. Believe it or not, you can definately overprepare, and you want to be natural!
Good luck to you on Friday! Let us know how it goes!