There a few basic things that you should do when (or even before) you receive an invitation for interview at a Dental school.
1) Think about, or jot down some answers for the most common questions, such as "Why Dentistry", "Why Dentistry as opposed to Medicine?", "Why this University?". (Some more examples of common questions can be found in the links below). Most Dental schools will ask a variant of these questions, and because they know that applicants will have been able to prepare for them, they will expect well-thought out answers to each of these that show a clear motivation for studying Dentistry at their University. Its best to be fully confident with your answers to each of these, or at least know what you're going to say, so that you can concentrate on the more difficult/unexpected questions that may be asked.
2) Read through your Personal Statement again. At some of my Dental interviews, the interviewers had my PS in front of them, and this is the only document they have to gain a glimpse of who you are. Therefore, expect to be questioned on certain aspects of your Personal Statement. For example, if you wrote that after observing a denture fitting in a GDP, you became aware of team-work needed in Dentistry and this prompted you to arrange work experience in a Dental Laboratory, make sure you know the basics about Denture fittings, and the roles of the Dentist and Dental technician. Interviewers will pick up on anything in your PS, and it may be a point you forgot to research further, so make sure that you know your statement inside-out and can answer follow-up questions on your work-experience, voluntary work, any Dental procedures you saw and your extra-curricular activities.
3) Read the University's prospectus again, especially the Dentistry page(s) and the pages about Student life in X City. This will assist you in answering the common question, "Why this University?". You should be able to draw upon the aspects of the Dental School, the course and the city itself which attracted you to apply there for a 5-year course.
4) Don't memorise your answers to questions! Simply have an idea in your head of what you want to say for the most common questions as listed below.
http://www.thestudentroom.co.uk/showthread.php?t=296040 - A very good source of the most common interview questions.
Below are listed links to some sample answers for different Dentistry interview questions: