Say thank you to your interviewer at the end and after by email or send a 'thank you' card by post.
Check your recent twitter posts are not unprofessional.
Do not use unprofessional terminology.
Don't be late. Plan for the unexpected.
Be genuine, cheerful, attentive, self-assured, forthright and to the point.
Take a seat, be calm, take a few minutes to set yourself ahead of going through the door.
Advice for those looking for employment:
"Read a paper or relevant blog that writes about the niche you're looking for a job in. This will make sure that you're up to date about movements in the field and will also be useful in an interview and shows the possible company that you want this job."