1. Playtest other's maps and take time to leave some comments. Not only does this help out the mapper whose map your rating, It also gives you invaluble experience you can use, to judge/improve your own maps. I cannot stress how important this is.
2. Model--not copy--really good mappers. By this I mean, find someone, anyone, whose maps you think are awsome, then study what they did, to achieve the creamy-goodness. It helps If the mapper your emulating has included the .RMF file, but it's by no means necessary. In my own experience, I've found really good maps--ones that really turn me on--are the result of exemplary texturing, and those that also have a simplistic elegance of design, that I don't see very often.
3. Read Every tutorial you can find, then read it againe.
4. If you can't figure something out after Googling and experimenting, ASK! There are many Vastly experienced mappers out there--not the least of which hang out at TWHL--who are 'dying' for new, stimulating questions.
5. Don't give up. When you've tried your best, and your maps still look like hot garbage, press on! Remember why you started mapping and keep trying. Keep your thoughts on the prize, as it may be as close as your next map away!
6. Don't be afraid of new things, but also don't blindly jump into things you don't understand, and expect great results. When I first started using VM, I got nothing but invalid solids and weird compile errors. After a while, when I knew a little more about the engine and hammer, I tried VM againe, and now I use it constantly--with some measure of success--!