Know the events that the organization sponsors, some recent projects, and the mission of the group. Emphasize your strengths, minimize your weaknesses. Everyone has weaknesses, but you do not have to highlight them in an interview. Stay focused during the interview. Keep your eyes on the interviewer.
Nonverbal communication is just as strong as verbal communication.
Watch your posture, too. It is important to go over what you know about the organization and why you want to join. Think about questions you could be asked and be prepared to answer them. Again, remember to be yourself. Answer how you want to answer, not how you think they they want you to answer.