Etiquette expert Jan Goss agrees—so much so that she always carries a baggie of small thank-you cards in her purse. “Never pass up an opportunity to leave a thank-you note.

The same goes for money, which some people believe is taboo to mention in a thank-you card. "You can write 'thank you for the monetary gift,'" she tells TODAY.com.

I really appreciated it." Or "Thank you for thinking of me this Christmas season. You helped make my holiday special." The message, like the examples above, should be simple yet sincere.