You need to be confident about your abilities when you are interviewing. If you are stuttering, breathing irregularly, wringing your hands, looking everywhere else but at the interviewer, chances are they will think you are nervous at best, lying at worst!
Make sure you are confident in your skills. Repeat them into the mirror, practice with a friend and have your elevator pitch ready.
Breathe! Practice breathing techniques that will calm you before the interview.
Organize your thoughts. Plan what your responses will be. there will always be that one or two questions that you didn't expect. Be prepared and stay calm.
Keeping your hands folded on your lap displays calm.
Keep your eyes on the interviewer.
Speak in an even tone.
Practice makes perfect. ALSO:
Anticipate (visualize) that you are NOT nervous. That you are calm and either way, hired or not, one more interview is just one closer to your next job!
Let me know if you need any help!