My favorite type of sketch is the 2 value sketch, or 2 value notan; here at the top.
I use a bold sharpie-marker to lay in the dark values of the sketch and leave the white of the paper for everything else. I skipped the mid-tones, so for every shape I must decide to push it to black or the other direction, to white. The 2 value sketch may take some practice but the design potential is huge. Suddenly, you will begin to see patterns and shapes form that you may not have been able to see in a 3 or 4 value sketch.
Compare the top sketch to the more conventional 3 value sketch below it. Both explain the light and tell the story but present very different pictures.
SideBar: I use the 2 value sketch in my own work because of the graphic quality it brings to the paintings.
In both sketches, I am very conscious of connecting the darks so that the viewer's eye can move across the composition with out a break. The final result was this little painting I did as a class demo.
The practice of translating what you see into small value sketches will help develop your artist's eye for VALUE.
What ever kind of sketch you choose to do, it will set you up to know where you are headed in the painting;
You master the work instead of letting it master you.
And we all know what that feels like - when a scene has just kicked your butt. . .
Go back and attack it with a value sketch.
Next time: TIPS on creating better value sketches.