If you avert your gaze, you may give the interviewer the impression of being shifty or unsure of yourself. If you crush the interviewer's hand, the pain will dim your lustre. Smiling sounds simple but is one of the most important rules of the interview. It sets the tone for the entire session, projecting you as a pleasant person. Make it a point to look at the interviewer directly when you are answering his or her questions.
Body language is also important.
Do not fidget. Assume a comfortable posture from the outset and avoid shifting your position or crossing and re-crossing your legs. If you are a smoker, by all means do not light up. If the interviewer wants to smoke, it is his or her prerogative. Know your resume thoroughly and be ready to elaborate on any point contained in it.