Handshake: A firm handshake is considered a sign of confidence. Take the other person's hand (don't use both hands), give a firm, but not crushing squeeze, and shake the person's hand up and down slightly, once. If you have sweaty hands, be sure to dry them before your interview.
Posture and physical distance: When sitting in a chair, sit up straight or lean slightly (don't slouch). If you will be crossing your legs, do it so that one knee is stacked on top of the other or cross your ankles.(Do not cross your legs so that one foot is on top of your other knee). Alternatively, keep both feet on the floor. Do not stretch your legs out in front of you or sit with your legs spread far apart, it looks too casual. When standing near someone, about three feet of distance is standard. Standing closer than this can be uncomfortable for others.
Arms and hands: You can "talk with your hands" to some extent, but do not do so to the point of distracting your interviewer. Sitting with your arms crossed in front of you can look defensive. Instead, try to have a more open posture. Don't fidget, play with your hair or pen, or bite your nails.
Eye contact: Look in the eyes of the person interviewing you. Looking down or away frequently gives a message of not being confident or being confused. Rolling your eyes up is considered a sign of disrespect. Don't stare at the interviewer; but look at him or her in the eye as much as possible.
Facial expression: Smiling is an important way of showing that you are a friendly individual and that you are enthusiastic about the position. Smile at the beginning and the end of the interview at a minimum. This can't be emphasized enough.
Our job is to get you in the door. Your job is to "get the job". Go into every interview with the goal of getting an offer. Be enthusiastic, upbeat, and positive. Show your interest. Do not leave any doubt in the minds of the people you interview with that you are interested.