Sent a Thank You card to one and a bottle of his favorite scotch, one a bottle of Jack Daniels, and one a big street sign that says Jimmy Buffett Blvd (he's a huge JB fan, we listen to it all the time in the OR). All of these guys were people that not only wrote me a letter, but also did mock interviews with me, and have let me shadow them throughout college. All were very appreciative. I think they like to know that they have helped, they all remember what it was like to be in your spot. 😉
P.S. - Talk to their nurse's they always have good gift ideas.