I think we all agree that sending handwritten notes is a great habit to get into. A note is always an appropriate and appreciated gesture if someone has made time for you or provided you with some assistance—or sometimes, just because. Keeping up with friends on Facebook is convenient, but sending a note is more personal.
But instead of getting generic notecards at the office supply store, consider finding something more unique on Etsy. Etsy is a website where crafters sell their wares, and there are a lot of hand-crafted notecards. I prefer something with a monogram, but look for cards that suit your style. (I keep a variety of cards on hand.)
When writing a note, keep it short. Two or three sentences are best. In a thank-you note, for example, say thank you, reference something notable from your interaction, and express your wish to reciprocate or to see the person again.
Expect to get thank-yous for your thank-you notes!