Here's a demonstration of variation to be found within a single thrumbnail, created from a square brush and selections in Photoshop. #147 is broken up internally three different ways, with no drastic departures from the original plan.
Lena astutely pointed out that I was creating "new lines" or unwanted edges into my stuff, and showed how gradients could fix this. I have to make sure that sharpness I add into my paintings serves a purpose (changing plane, etc.) and isn't just there for its own sake.
Painting under lines hasn't been working out for me -- was always getting away from the spirit of the drawing -- so I decided what I needed was something halfway between lines and shadow data. By working under/over/within this "pattern mask" (i.e. the darkest darks) and keeping the original drawing nearby for comparison, I came up with this.