Practice, practice, practice.
In order to learn some anatomy I set out to paint a man in Photoshop.
I started with a stick man. Seven heads high.
Then I used the vector tool in Photoshop to place some volumes.
... after which it was quite a lot easier to produce a better sketch.
Then I could start with his flesh.
But I realized it might be a good idea to actually draw the light source before putting in any details.
With the light there, details was fun and games.
To make him a little less flat I added shadows.
Finally I hid away some flesh layers and got a semi invisble man.