This is more important than many people give it credit for. As well as considering bad/off-putting habits which you may have, you should try to adhere to the following:
Ensure that your handshake is firm.
Always maintain a good posture, comfortable yet assertive – do not slouch!
Make sure that you listen as well as you speak. Make sure that the interviewer is aware that you are taking on board their comments, whether by nod or timely interjection, and if there is more than one interviewer present then switch your glance between them at regular intervals.
Whilst gesticulation can often help to get a point across, make sure that this is limited as it can also suggest nervousness.
Integrity and Trust
Dealing with Ambiguity