I have had several interviews now (unfortunately I haven't got any yet!)
But I normally do three things- brush up on my skills, really read up on the project- read some literature around the project- as they are sure to ask you some questions on it, read about the university and the
supervisor- double brownie points if you read some articles by the supervisor, also double brownie points if you email them and ask what literature you should read ahead of the interview
also formulate 3 or 4 really good questions to ask- a good tip is to find out who is on the panel then you can formulate questions for everyone on the panel.
What is the interview for? Good Luck :-)