These days there are so many instantaneous ways to communicate that taking the time to hand write a note and put it in the mail feels rather substantial. I try to write thank you notes for the following things:
gifts- especially if someone MAILS me something
staying at someone's house
a thoughtful favor
a meal- traditionally called a bread and butter note, I save these for relatives, older people or someone I don't know well (a friend of a friend etc), because if I wrote thank you notes every time i had dinner at a fiends house i would be writing thank you notes ALL THE TIME.
thank you note from near modern disaster
emily post on thank you notes