Check if there is any interview training or opportunities for dummy interviews on offer within your institution. The Careers Office is a good place to start.
Anticipate questions and plan your answers - check out a few of the more common interview questions and some hints.
Re-read your covering letter and CV/Resume - if you were reading this what questions would you have for the author - sometimes there are some obvious questions.
You will probably be given the opportunity to ask questions yourself - think about what you would like to know about the company or job, plan a few questions but don't turn it into an interrogation.