Just relax and be positive. People want to like other people, particularly in these sorts of stressful situations. You're a smart, capable person; I think you needn't worry about saying the wrong thing.
I try to remember that the interviewer probably doesn't like being there either.
I prepare as much as possible by doing what you've done; learning all I can about the company/organization. I also try to answer, for myself, why I want the job, where I see myself in a few years, and why I would be the person they are looking for.
I usually come up with crap answers but it gets my head in the right place and once I am fully saturated, I can let go and just be relaxed and be myself.