Think of body language like not playing with a pen, sitting upright, not slouching and speaking clearly.
Be cautious to talk poorly about your team mates, boss or industry.
Do not go off topic. Keep your thoughts on the things you are good at.
Do not be worried to declare you don't know an answer.
Check that your recent twitter posts are professional.
Advice for those looking for employment:
"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."