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 luster. 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 do, it may give the interviewer a message that you are uneasy or nervous, which can translate into the perception that you are trying to conceal something you do not want the employer to know. Know your resume thoroughly and be ready to elaborate on any point contained in it. But you have to be in mental command of all of your important accomplishments.