I used to send handwritten notes all the time, but - yeah - as another seller said once I reached a certain amount of sales it wasn't practical. I didn't have the time and my hand kept cramping up. What I do now is I have a word file saved with sort of a generic thank-you, description of how much their purchase means to me, coupon code information for any repeat business, my facebook and twitter info, and request for them to leave feedback if they are happy and contact me with any questions.
I write their name at the top and sign my name at the bottom. If we have had correspondence via convos or emails prior to the sale, I usually add a short note, and if the purchase is a gift and the buyer requests it I often include a handwritten message on a small notecard.