How to learn composition?

I rarely seem to be satisfied with the photos I take. There is always something wrong with them. I know how to judge a great photo while looking on my computer, but out in the field I just seem to miss obvious techniques and opportunities. Most of my photos are of the "That's nice, but so what?" type. Whereas many other good photos leave nothing up to the imagination, "Oh, right, that's what the shot is about". My composition just doesn't seem well developed.

I can take a technically good photo but then I go to my "artist" friends' facebook photos, using a tiny P&S, and their shots are just so much more interesting. Maybe it's like how someone else's lunch sandwich seems better than your own.

How did you learn your composition?