pyromairon 

Commission for Vanari (twitter)

You need to make a few simple sketches, where:

1. The foreground is light, the middle ground is gray and the background is dark,

2. The same thing, but in reverse,

3. Try random colors. For example, the foreground is dark, the middle ground is light, and the background is gray.

4. Black in the middle and light around? Why not?

5. And etc.

First, try simple forms like in my example, and then try to complicate them by adding details and then even more details.

Here, I tried to summarize the exercise in one picture.

Try to spend on those sketches 10-15 mins max, the less the better. It trains precisely the quick basic sketch of an idea. Because concept art is basically about an idea, not about a gorgeous render (which, of course, you can always add and make concept into illustration)