What does the etiquette book say about this? I had 2 professor write me a LOR for a summer internship and the same 2 write LORs for transferring. I feel quite indebted to them since LORs are not easy to write. I remember people talking about this when I was applying out of undergrad...but I guess they took away the search option on this site?
I always bake a banana bread and send it to them in a nicely wrapped package with a note (cookies also work well). That way they don't feel like there's been a transaction involved but they know I'm appreciative of their efforts.