Think of body language like not playing with a pen, sitting upright, not slouching and speaking clearly.
Try not to oversell who you are.
Sit in the car, be calm, take some time to set yourself ahead of entering reception.
Think about your responses.
Be sincere, upbeat, self-assured, direct and succint.
Do not be fearful to declare you don't know an answer.
Say thank you to your interviewer at the end and after by email or send a 'thank you' card by post.
Suggestions for those looking for employment:
"Before you have the interview find information about the company. Get to know what the firm does, who their competitors are, and their principles. An interviewer won't take notice of your input if you havent researched how they operate."