Be on time - or, better yet, EARLY! Arrive at the interview no later than 15 minutes prior to the scheduled time. Find a bathroom where you can do a last chance check of your appearance. Look to see if there is anything in your nose - gross, but look, see if there is anything in your teeth. For women - is your slip showing? For men is your zipper up on your trousers? What a nightmare it is to get back in your car only to discover one of the previously mentioned items was not discovered!
Smile warmly when you enter the room where you were asked to report. The receptionist is a very important person for you to be friendly and warm with. They DO have influence over the decision so do not blow it by being rude.
Again, when you are introduced to the interviewer, smile warmly. Make and maintain good eye contact and offer a firm handshake. This goes for both men and women, don't be a "cold-fish" handshaker! Practice if necessary.
Usually the interviewer will try to make you feel at ease by making small talk with you while escorting you to the location of the interview and while getting settled into the room where the formal interview will take place. Follow the lead of the interviewer on the topic of the small talk and participate.