I've always liked the idea that what comes around goes around. My theory is that if you start sending more personal mail, you'll almost certainly inspire at least some of your recipients to do the same, thus transforming your own mailbox into a mailbox of delight.
But I can't wait that long, I hear you say. I want a mailbox of delight NOW! If that sounds like you, you might want to think about signing up to Postcrossing. Postcrossing was founded more than 5 years ago by Paulo Magalhães who describes himself as a "geek with a passion for postcards". The goal of the project is to allow people to receive postcards from all over the world, for free. The idea is that if you send a postcard, you will receive at least one back from a random Postcrosser from somewhere in the world. This beautiful postcard recently journeyed from Germany to Taiwan:
The more you send, the more you receive. What's not to like about that?