Smile and maintain good eye contact throughout.
Dress conservatively / appropriately for the interview.
Prepare a list of questions to ask.
Answer questions with more than just a "yes" or "no" - always try to back up answers with examples which illustrate your skills and how you could contribute to the company.
Let the interviewer bring up the subject of salary, when asked about salary always back up your answer with "although salary is a factor for me, the nature of the job and organization I work for is very important too."
Don't criticize your current/previous employer.