Last year a teacher started something at our school that I thought was so cool, I had to do it to. I've done it a few times, and this last time I got a great response from one of my students.
I've done it in advisory (we meet once a week for 30 minutes), but I've also used it when I've had an extra 5-10 minutes in math class. I ask my students to write a thank you note (on paper I provide, with markers I provide) to a teacher in school which I then put in the appropriate mailboxes. I mention that it shouldn't just be, "thank you". It should be genuine and indicate something they appreciate about the teacher, and it doesn't have to be "saga long".
Anyway. This last time I did it in advisory, a couple of students were balking. I tried to persuade them with, "it really feels good when you get an unexpected positive note or compliment or such from someone. Think about how you're making someone feel." One student finally, grudgingly wrote a note.
The next time in class, he talked to me about it and said (with a grin on his face) that that teacher had come up to him and said she was having a horrible day, and then she got his note and it cheered her up immensely.