Some advice from a cheap pack of Thank You cards, uncut and unedited:

Some Idea Starters for Saying "Thank You"

1. You always do the nicest things.
2. What you did warmed my heart.
3. Thank you for remembering me in such a nice way.
4. Your thoughtfulness was sincerely appreciated.
5. Your kindness meant a lot.
6. So nice of you. So appreciated by me.
7. Knowing it came from you made it even more special!
8. I'm so very grateful.
9. Think about how you would say "thank you" in person, and your sincerity and warmth will come through in your note.

Also to note: for as many logical reasons as this cheat sheet is not ridiculous, all of the "idea starters" are in small caps.