You can never go wrong with a nice, simple thank you note. Handwritten as mentioned. It shows our appreciation and lets others know when they have done something special that we appreciate. I just got done writing some myself for some professors who wrote letters of recommendations for me for a scholarship. It is something they didn't have to do, and I wanted to let them know how very much their thoughfulness and kindness meant that that they did. You never know how people touch you otherwise, it can mean a lot.