Do not go off topic. Keep your thoughts on the things you are good at.
Check that your latest twitter and linkedin messages are positive.
Do not list items on your CV. This interview is an opportunity to submit the positives you have that your CV just can't depict.
Learn regularly asked questions and rehearse your replies.
Make a Google Alert on the firm to know latest developments.
Try not to talk poorly about current team mates, directors or suppliers.
Suggestions for those looking for employment:
"Proofread! When you are proofreading your CV, have one or two other people read it also before you show it to a potential employer. Sometimes when you work on something for a long time your brain can automatically make changes and correct what you are reading without you even realising it. Spell check is not infallible and cannot discern between words like wood and would for example."