Don't go off topic. Keep your brain on the things you are good at.
Say thank you to your interviewer at the end and after by email or send a 'thank you' letter by post.
Consider your responses.
Create a Google Alert on the employer to know latest developments.
Eat properly before - and no stinky ingredients!
Never be late. Prepare for getting lost.
Advice for Job-hunters:
"Read a paper or relevant blog that concentrates on the area you're looking for a role in. This will make sure that you're relevant on updates in the market and will also be useful in an interview and tells the company that you are serious."