To address some of the above comments:
1) Always send a thank you. I can't imagine it's an automatic ding if you don't but it's just common courtesy.
2) There really is no format that you need to follow but mine usually come out like:
Dear Interviewer,
Thanks for talking to me about this school. It was interesting to learn that XYZ. As we discussed, I am eager to ABC if I enroll. Hope you enjoy the rest of your semester.
Very simply, say thank you and touch on a few things if you can.