Nice post a few weeks ago from Church of the Customer (found them today on Seth's blog, good stuff) about the art of the thank-you note.
A handwritten note is about the cheapest and easiest way to make a memorable micro-connection. But when you're going to be memorable, take an extra minute to get it right.
I'd add two suggestions to Church of the Customer's five.
One, write in the language that you speak in. Write "help" instead of "assist," for example. Clean the gunk out of your verbiage (and your thinking) and write in simple, clear words.
Two, (and this is an extension of CotC's advice) tailor the tone of each note to its reader. If the note is to a 60-year-old professional, signing the note "love ya, mean it" might not be the way to go.
Along the same lines, give some thought to what your note cards and even your ink color are saying about you. Consider how your notes can become an extension of your brand. Not through logos or brand colors, but through how they communicate your promise. Are they formal or lighthearted, geeky or warm? The most expensive or formally "correct" notecards might not be the ones that back up your brand's promise. The right answer is the one that deepens the communication.
A handwritten note is remarkable precisely because it is one-of-a-kind and takes thought and care. Make the most of it. Turn a one-minute task into a three-minute one, and watch what happens to your relationships.