Although this probably isn't news to you, here are some basic tips...
Be sure to maintain eye contact. If you are uncomfortable with it, try looking at the tip of their nose(and pray they don't have a booger hanging out). Probably a good idea to do a bit of stretching/flexing before you arrive to relieve some tension. Firm handshake, good posture, yadda yadda. When sitting during the interview, place your arms on the armrest or on your knees rather than crossing your arms or twiddling your thumbs.
When he or she asks you a question, try to take a second before answering the question. Also, try to use the right words. There is a difference between answering a question with "yes" and answering with "yeah" or "sure".
First impression can play a part in how people perceive you, so try to have a smile on your mug when they first see you in. Not some maniacal grin, unless you think that's what they're going for. :) Don't be afraid to smile.
Good luck with the interview.