How I did it: In a moment of nostalgic indulgence I bought a box of 100 postcards depicting Penguin book covers.
I spent several months sifting through them and smiling and remembering and putting them back in their box and into a drawer; each time a few cards would remind me of particular 43Ters.
After a while I decided it was silly to keep hold of them, made a goal and starting shamelessly begging and pleading with people to let me send them a card.
50 people said 'yes' and I decided to keep and frame the rest so I considered the goal complete (note careful wording of goal ;p).
Lessons & tips:
Don't be afraid to hussle, hassle, guilt-trip and pester in order to find recipients for your cards; an imaginative approach can win people over ;p
43Things and 50 lovely 43Thingers
The Post Office
Various Worldwide postal systems