Via Fast Company's Innovation blog, I found the top 5 ways to thank your customers. I'd like to draw your attention to #3:

3. Use your data base for them, not against them. Select a loyal customer at random and surprise them with a free gift or a discount. Refrain from congratulating your customers for being loyal with yet another sales pitch. Just send the gift.

Fantastic. My phone company recently used a similar ploy. They called and discounted my monthly bill by $8, simply because I'd been loyal. This outbound loyalty campaign made me feel better about being their customer -- and likely ensured that I wouldn't feel the need to shop around.