This sounds strange, but sit a day or so beforehand and just visualize yourself kicking ass. It helps. It's something athletes do before a big game/event to focus on what they need to do to succeed. Take note of what your "daydream" self is doing. Posture. Affect (facial expression). What's running through "your" mind? Model that as best you can.
Another thing to consider is that the person who is interviewing you may not be much older or may be inexperienced at giving interviews, hence he/she will be almost as nervous as you and totally relates. If the person is much older or experienced, then they will have seen many a jittery person through the years. If you come across as *too* confident they may find you arrogant or cocky and not want to hire you. A bit of awkwardness can be endearing. Just don't throw up on them or anything.
Good luck! You'll do fine!