The main lesson I've learned since doing these two years ago is that it's better to use proper watercolor paper in postcard-form than attempt sketches on the slick, already-printed-on backs of commercial postcards.
There's no need to think up words, and even if you sketch the same thing for multiple people, each version will be unique.
I promised you bridges, didn't I?
(I did quite a few more than these, but didn't have a chance to scan them all before they were mailed.)
