What is the best thing you can do to prepare for a job interview?

Always sleep well and do not panic, many people try to exaggerate about there self and many crack under easy question.
Try also to think that it is at the end a job interview and it is not the end of the world.
Browse the internet and search for general information regarding the job that you are applying for.
The employer of course posts a job advertisement that show the job role or tasks, either ways read your CV couple of times and prepare yourself for questions about the information written in it.
Also you should do whatever activity that makes your relaxed before the interview, I always listen to my favorite music and smoke before I go to an interview :)