Job interview always makes people tense because how you express yourself will determine whether you'll get the job or not. The most important thing you should never do during the interview is panicking, make yourself as comfortable as possible. Being comfortable can only be achieved when you report for the interview 30minutes before so that you get familiar with the environment.
Also, wear neatly and decently. Your dressing code speaks a lot about you. For the ladies, its advisable not to put much make up. Have some courtesy especially during the interview, the words your utter and how you express yourself.
Finally after the interview say thanks and lock the door behind. Generally, you need to be cautious of how you express your with a lot of confidence, that's the only secret.