they dont read the thank you cards that are sent by snail mail... the secretary writes your name down for some of them, but they really get to many so they dont have time to read them and they rarely see them... i know this because i sent something to a school and she said, oh no dont send it snail mail because it will get mixed up with all of the thank yous that we are getting and it will go to the secretary... i joked with her and said so you dont actually read them and she said, "of course not, no one does, there are just too many..."
makes sense... but i would say send an email stating thanking the person add something personal and then finally at the end say expect a proper thank you in the mail... blah blah blah you know so they know that you put the effort in and they will be looking out for it when you actally send it... (oh yea and actually send it by snail too)