Pay attention to the following.

Sit erect comfortably without craning your neck. Do not slouch.

Look attentive, keen and interested.

Talk clearly, maintaining a pitch that is comfortably audible to the person(s) around.

Do not get overexcited even while describing your achievements and strengths.

Listen to the queries attentively, constantly maintaining polite eye contact with the interviewers.

Nod your head to show that you are listening, interjecting appropriately with Yes Sir/Madam, absolutely, definitely etc.

Lean forward a little as you speak and backward as you listen.

Do not fidget, touch your face, or shake your legs.

Keep your arms either on your sides or in your lap. Do not fold your arms, as it is a sign of rudeness.

Use short simple sentences while talking.

As You Enter

Depending upon whether you are being interviewed by one person or a panel of interviewers, greet politely Good Morning/Afternoon or Evening depending upon time of the day.

If there are more than one person then address them as Sirs/Madams and try to encompass them all in your greeting.

You will be offered a seat. Do not sit down unless you are asked to.

Facing the Interviewers
 
As you sit across the interviewer , look confident and relaxed. In most cases the interviewer  himself will try to put you at ease. Believe that they are there to let you prove your worth and mean no harm to you.