So you want to become a good mapper? Then start by learning from others. I've collected a few articles here. Most of them I've read long ago and I'd say, they're helpfull and provide insight on the subject of level-design. So, why not take a look?
What is level design?
Common sense vs. level design
Half-Life Editing Tips
Design Inspiration
Detail vs. Design, part 1
Detail vs. Design, part 2
Feel free to add more articles, I'm sure it would help others. And of course, I'm interested too...