First... Read the interview feed back section.
Second. BE VERY PREPARED for them to ask you "what do you mean?"
For example. They ask, "why do you want to be a dentist?" You ramble off your perfect response that you have practiced. They say "How does that make you want to be a dentist", or "I don't understand." If you are not ready or capable of making off the cuff remarks you will not do well.
The interview should be pretty light. Nothing too heavy.
Practice your responses to yourself. Make sure that they fully answer the question and they are concise. I practiced 10 or so questions and timed my self so that each was no more than 2-3 min long.
Smile, be honest, be funny... and EYE CONTACT!!!!
Good luck.
-C