Make sure you look presentable. Dress business-professional even if the company has a casual dresscode. Make eye contact, and make sure to smile from time to time. First impressions are very important. If you look put-together, you will have a better chance over someone who is dishevelled.
Other than appearance make sure you have a copy of your resume with you. Take notes and seem interested. One important aspect of an interview is the time you get to ask questions. Do some research on the company, and ask specific questions based on your research. Ask about their company culture and the position. Many interviewee's fail to ask questions, and don't get the job for that sole reason.