Be ready for conversation with knowledge about the company, responses to common interview question, and questions of your own.
Don't act like you know everything or you will seem arrogant. But certainly read the organization's website and relevant news so you can talk intelligently about the company.
Practice your responses to typical questions and prepare stories about your recent accomplishments, particularly ones that illustrate how you make you a great fit for the organization. Finally, put together a list of questions to pose when the interviewer asks if you have questions.