Prepare and over-prepare:

It is assumed that you don't go in with egg on your tie, spinach in your teeth, or without a thorough knowledge of the organization and position for which you are interviewing. Beyond that, there's an important principle that will enable you to be much more confident. It's called, "over-preparing." It goes like this: Plan your strategy--your answers to all the possible questions you may be asked or the challenges that may be thrown at you--and then practice, practice, practice. Role play and repeat your best responses until they are entirely natural, until they simply roll off your tongue with the apparent spontaneity that comes only with successive repetition.

From: http://topten.org/public/AB/AB8.html