Code review. Oddly enough, it wasn't so much the issues that were raised by the reviewers that made me a better programmer (though they certainly helped), but the knowledge that every aspect of my code could be inspected, and I'd be expected to fix it, that forced me to double- and triple-check my work before sending it off. Since that time, even when I'm working on personal projects, I find myself coding defensively, and will often write sections of code a couple times until I'm satisfied that it is unambiguous, correct, and clear.

This has helped immensely when I've gone back months later and tried to figure out what I was thinking; now, it actually makes sense!