Here is the picture I plan to paint, with a sketch of the same thing. Notice the grid lines running horizontally and vertically across both the picture and the sketch. These lines help me keep my perspective--whatever is in one square of the picture should end up in the same square of the drawing.
The extra lines on the drawing indicate where I'll have a drastic color change for the horse's blaze, his nose, his lips (contrast). So when I get into the flurry of painting I won't forget where those lines should be.