Kevin Hale the founder of Wufoo recently gave a soapbox talk where he mentioned that his entire team sits down once a month to write and mail thank you cards to every paying customer they have. Building such a relationship with their customers does wonders for retention and loyalty. You can see how genuine this card is, you sense that the team is genuinely thankful to the customer for using their services. They truly care about maintaining and developing a relationship.
Here is a great example of what happens when a people write thank you cards without caring about the people they are thanking. A few months ago a friend mentioned to us that ZURB should speak at Internet Week conference in New York City. We checked out the conference, seemed like a neat event. After sending the proposal in we never heard anything back from Internet Week. We figured no biggie, they're probably crazy busy and will get back to us. They never did. After two weeks however, we started getting emails about the latest news of the conference. They got pretty annoying so we ended up putting a spam filter on them. Yesterday we received a hand written thank you card from Internet Week thanking us for participation in the conference.
If you want to thank someone do so genuinely, otherwise you'll lose your customers.