Be aware of your body language. Look enthusiastic, alert and focussed on the interviewer. Offer a firm (not crushing) handshake. Sit upright and slightly forward, this indicates interest and energy.
Make eye contact as much as possible. Looking someone in the eye shows that you are interested in that person.
Be courteous to anyone you meet during the interview process, no matter who they are or what their role is in the process. Anybody's impression of you can have an impact on a final decision.