Even if you're nervous, be sure to listen carefully. Ask meaningful questions. Answer questions in terms of the position or the company. Describe how you can apply those skills successfully in this job, and how you can be an asset to the company. Employers don't like to be told how to run their company by someone who isn't yet part of it.
Attitude is half the battle.
A positive, upbeat applicant always does better than a negative one. Aptitude is also important. If you want the job, indicate your interest. Conclude the interview with a summary of your qualifications and your interest in the position. Ask what the next step is in the interview process.