Do lean forward a little while speaking - it will make you physically and communicatively closer to your interviewer. Don't stick your hands in your pockets - it's not correct etiquette, and might look disrespectful. Don't go about waving your hands as you speak - it looks common and impulsive. Do use your hands sparingly to emphasize certain points as you speak. Do cross your fingers while placing your hands in your lap, or on the desk in front of you.
Many people admit the first things they notice about others are their eyes.
Don't look down at the floor - it looks insecure. For that matter, don't let your eyes wander around the room too much, avoiding your interviewer - that looks insecure, as well. Do look your interviewer straight in the eye, for the better part of your interview. Do look briefly at the documents you're holding - or better yet, actually scribble something down, if you must gather your thoughts and feel uncomfortable staring at your interviewer while doing so. People often look at facial expressions to determine how others react to situations, what their emotional condition is, and what they're about to do.