Well, in addition to what's been mentioned about using very little paint, there's two things I can think of to mention: Paint across the cracks you don't want shaded ( not along them ), and use a very light touch.
If you're still not getting the desired results ... practice, practice, practice.
Hairy's telling the truth. I've been painting for a long, long time. And I absolutely sucked for much of that. I was very active a few years ago, and that was basically when I learned to paint passably well.
I will add one thing, though. In addition to just practicing, it's also a good idea to look up guides, and ask around. You'll learn how to use techniques properly, and you'll learn new, useful techniques.