Imitation is definitely a valid approach to developing your eye for technique, but you should also actively investigate the principles and theory of design. Spend a little time each week reading about things like color theory and experimenting with what you learn on your maps. Maybe read about composition—what's the "rule of thirds" and how can it make a map look better?

Obviously, you don't want to spend all of your time just reading about how to make art, though. For every hour you spend reading about something, you should spend at least four hours making something. I firmly believe that everyone can make good art if they spend enough time and energy practicing at it.

Wikipedia has an excellent article to serve as a jumping-off point for research: Design elements and principles - Wikipedia, the free encyclopedia
There are also tons of design blogs out there, in addition to the more specific guidance you can get here.