Sit down, be calm, take a few minutes to ready yourself ahead of going through the door.
Eat properly before - and no stinky ingredients!
Say thank you to your interviewer at the end and after by email or send a 'thank you' letter by post.
Do not be worried to say you don't know a response to questions.
Interact, perhaps, with the firm's twitter profile.
Make a good first impression, offer warm greetings with all you speak to. Stand up, make a smile, keep eye contact.
Put on a nice suit. Be smart and neat.
Advice for Job-hunters:
"Before you apply, make sure you unearth information about your prospective employer. Understand exactly what they do, their sector, their competition and principles. Stay on top of any issues or news in the sector."