Practice Makes Perfect
The goal is to be relaxed during your interview. If you are relaxed, then you will appear confident. Confidence is then interpreted as aptitude. Your employer will assume that you are a good fit if you are confident in your abilities. To ensure that you are relaxed as possible, you should practice your interview skills as often as possible in the days leading up to your big interview. The best practice is actually going on several job interviews, but that can be a tall order. If you need to, you can get a friend or relative to interview you. It may not seem like it, but a small amount of practice will help work out those pre-interview jitters.