You can't do your best in an interview if you haven't spent time preparing. Do your homework by researching the company. Start with an Internet search: Look at the company's Web site, read their mission statement, get hold of an annual report. What's their history? Who are their competitors? Has the company been in the news lately? Who are the important people in management?
Study the job description carefully, and review your résumé. What are the areas of your experience that might have prompted them to call you in for the interview? What experiences and skills should you emphasize? If you can, stop by the company's office to pick up brochures and observe the atmosphere.
Practice your interview beforehand. Have a friend ask you questions from a list of probable queries. Don't memorize your answers, but be confident about the key points you want to make. If possible, it can also be a good idea to learn something beforehand about the person who will interview you. You might try searching on Google, Facebook or LinkedIn.