Brush up on what the job description entails. Know the job like you've worked there for years ... they'll pick up on the confidence. Got the job description? Know anyone else in the job who you can talk to?
Be able to give examples of previous experiences (working, studies or not) to demonstrate skills in key areas. Demonstrate a willingness to learn in the areas you know are your weakest. Be able to identify (in relation to the job) your SWOTs (strength, weaknesses, opportunities, threats).
These are the things I look for. I could go on, but keep it simple: know the job, prepare for questions, relax.