I also sent thank you cards to my interviewers (blank inside, painting of Pebble Beach Golf Course's 18th on the cover). I thanked them for their time, picked out one SPECIFIC thing we talked about and reiterated my feelings on the subject, and finally included that their school was my first choice (because it was).
I did it informally (handwritten) and used a card which said something about me that they could easily remember (I'm a bigtime golf fanatic) because I WANTED TO STAND OUT somehow. If you can't do it with grades or test scores or EC's or LOR's...do it with personality! :wink: