How can I prepare for my interview?

Although it is true there is no 'cheat sheet' for the interview, there are definitely ways you can prepare:
Personal Statement:Make sure you know your personal statement inside out, and be prepared to talk about anything you have written. Interviewers can pick up on any detail, and you don't want to look like you have lied! So if you say you are really interested in the Ottoman Empire for example make sure you know something about it! What about it in particular are you interested in? Have you done any reading around it?
Be interested in your subjectInterviewers want to see that you have an interest in your subject outside of what you have learned in school. So, do some reading into things that you find particularly interesting. If you are interviewing for history, read up on some historical ideas or time periods that you wanted to learn more about, and think about any interesting exhibitions you might have been to recently - what did you like about them? If you are applying to do a scientific subject, this is perhaps slightly less relevant, but interviewers do want to see a keen interest and a capacity to think about concepts outside of what you have been taught.
Be prepared to engageWhat interviewers really want to understand is how you think, and how you approach problems. They will ask you difficult questions that are sometimes deliberately out of the range of your knowledge. This is not to trip you up, and you are not expected to know the answer. This is intended to see how you approach learning. It absolutely doesn't matter if you get the question 'wrong'. What does matter is that you try to answer it- in any way you think you can. Feel free to take your time, and think before you reply, but make sure you do answer. An interviewer can't find out much about you if you don't try to answer their questions. Just give it your best shot, and be prepared to think creatively!