There is a bit of a theme here that I'd like to highlight, and that's variety. Yospeck mentions a job as a freelance illustrator. That probably did a lot more for the mapmaking than merely making maps. The principles of design apply to any kind of visual art, and something you learn while taking photographs, for instance, may improve your cartography. Since I started working as a visual effects compositor, I have found that I have been able to offer much better critique here, even though I haven't made an actual map in quite a while. And when I do eventually get around to making a new map, you can bet that it will be substantially better than my last, even though I haven't touched cartography in quite a while.