When I was younger, my mom insisted that we wrote a thank you note any time we received a gift. It didn't matter how large or small the gift, you had to write the person a thank you note. There was a set time frame in which this had to be done and this was exactly two weeks. You really only had one week because if you hadn't written the note by the end of week one, Mom was giving you "the eye".
I am now a grown adult (or pretend to be!) and I still write thank you notes. I still adhere to the two week deadline and I still make sure to write in my best handwriting & in cursive! As a mom, this is one of the lessons that I have shared with my daughter. I explained to her, just like my mom explained to me, that a thank you note is a meaningful personal way to show your gratitude.
Many people have gotten away from personal letters, cards, and notes. I believe that there is something touching and priceless about receiving a handwritten note. If it has been some time since you've sat down and written a thank you note, take a look at chapter 7 of Writing Notes with a Personal Touch.
The last line of the chapter states "A written message that expresses the depth and sincerity of your gratitude for whatever favor or gesture takes on a life of its own and can be treasured by the person who receives it".
Interested in other books on thank you notes, letter writing, and etiquette? Take a look at the list I've put together.