I absolutely LOVE Nathan Fowkes' gouache studies. If
you haven’t seen his amazing work, check out his land sketch blog:
http://nathanfowkes-sketch.blogspot.ca/
(his life-drawings and paintings are mind blowing too).
I was inspired to study his sketches by doing some
quick digital studies on my Nintendo DS. I don't have the full range
of tools that's afforded with Photoshop (layers, undo's, brushes,
etc), so it's a nice exercise :)