Your main goals as you approach the interview are to give a positive impression of yourself and to establish rapport with the interviewer. Smile as you greet the interviewer and use his or her last name with a courtesy title. Don't use the person's first name unless invited to. Maintain eye contact.
Shake hands with a firm, confident grip. Don't sit down until invited to do so. Pay attention to your body language. Don't slouch -- it's better to sit straight up than to lean against the back of the chair. Don't fold your arms, cover your mouth or look away frequently -- these gestures can seem defensive and evasive to the interviewer.
It's also best not to tell jokes. Humor can be a good way to break the ice in a social situation, but it can be dangerous in a job interview. You want to come off as professional, not flippant. You shouldn't offer your opinions about non-work matters like politics or social issues.
Don't talk too much or too fast. Pause before you speak, and show that you're a good listener. One good tactic is to ask a question that lets the interviewer tell you something about himself or herself, such as when he or she started with the company. It shows your interest and puts the person at ease.