Q: What is the proper way to send a thank you note?
A: A thank you note should be written on a note or card bearing your name or monogram, rather than the words “thank you”. This is a common misperception due to the influx of thank you cards in stores, but etiquette dictates that a card should be personal and not look mass-produced (i.e. “less special”). I would make an exception to this if, for example, you found a completely awesome “thank you” style card like the one from Old Tom Foolery that says, “I’m forever indebted to you.*” (*By forever I just mean until you finish reading this card, then we’re square.)
In any case, the card or note should be personal and heartfelt, at least several sentences long, and should always follow a gift or an invitation (particularly if you’d like to stay in that person’s good graces and/or be invited back again sometime). In short: it’s a nice thing to do, so do it.