Body Language

Do's

Ensure a firm handshake. A firm handshake shows confidence in yourself and your abilities.
Walk slowly, deliberately, and tall upon entering the room.
Maintain a high level of eye contact throughout.
Remember not to be seen to be staring. Look away occasionally, looking confidently and calmly to the right or left; never look down.
Listen.
Be alert and enthusiastic - it's often a deciding factor in employing candidates. An indifferent attitude is instantly recognised as "I don't care if I get this job."
Smile, nod, and give non-verbal feedback to the interviewer.
Do not hurry any movement.
Relax with every breath.

Don'ts

Have a poor/limp handshake.
Display laziness.
Be aggressive or act in a superior, conceited or overbearing way.
Have a poor voice, diction or grammar.
Look distracted, look down or avoid eye contact.
Talk too much. Answer questions as asked, without being abrupt; expound only to the point that the interviewer has a clear understanding of what you mean.
Lose concentration or attention.