Research the organization and position before the interview. Prepare questions to ask the interviewer that reflect your knowledge of the position and company. Write down the time and place of your appointment, in addition to the name of the person interviewing you. Get good directions, and visit the site prior to the interview. Wear professional attire that is appropriate for the position.
First impressions are important.
Do your homework on the company and on the role the job has within the organization. Find out before you show up whether you're likely to have to take a test on skills, abilities, etc. In any case, practice the interview before you go. Know what you want to stress, what you don't, what your strengths and weaknesses are. The better prepared you are, and the more you practice, the less you're likely to be nervous and the better the interview will go.