Here's a brief primer on the surprisingly effective handwritten thank-you note:

1. Spell the recipient's name correctly (doh!).
2. Thank the person for choosing your business. If they shared a specific reason why they choose your business of why they like it, reaffirm it. For heaven's sake, though, don't turn it into a sales pitch.
3. Include a personal detail about the recipient that you picked up on. Prove that you were listening. Humanity is a good thing in the antiseptic world of business.
4. Open the door to feedback. Whether the recipient provides it isn't the point; it's the idea that you're passionate about creating a recommendable experience.
5. Be authentic: Include your full name and contact info -- email and/or phone. Or a business card.

(Slightly) more here