If you have access to small children, make sure they write thank you notes, because a) Adults love thank you notes and will buy your children even more gifts b) A small but vital archive of their handwriting will be established and c) It will make you as an adult realize that thank you notes are amazing and should be sent all the time.
For the Adult version you have to bite the bullet and invest in some good cards. These are expensive. Get over it. Buy them anyway, and in bulk.
I like a nice blank, not overly decorated card. This is much more useful than cards with images on them, because you don’t want to send condolences for death or a thank you to your boss on a Klimt sexy-kiss card.
I like these Crane & Co. ones very much.
Note: my mother totally gets away with image-based cards (which she buys in museums) because she is a visually-oriented thoughtful person. For instance, she really has a way with a good Wayne Thiebaud card, or a more dicey feminist artist card. But I don’t know if mere mortals can get away with this strategy.
But my husband, who comes from good breeding and knew about thank you notes while I was still finger painting with my pudding, likes a good post-card in place of a thank you note. He buys very weird but strangely neutral cards whenever we visit historic homes or pretty towns.
This historic house post-card has been a real winner for all-occasions notes:
I don’t even know where I stand on monogrammed stationary. It seems like a nice gift (both to give and receive), but it also seems like you are pretending you are Queen of England.