A while ago I came across the remarkable work of Jiyu Kaze. Researching how he made these amazingly 3D looking images, I came across a method of painting that was basically like reproducing one of the many passes 3D models go through called ambient occlusion. The end result wasn't something I would want in my work (though I had seen Sam Nielson use it really effectively in a much more painterly style), but I love seeing if new methods can teach me something about the way I like to work.
I didn't save out too many wip images, but it was a really interesting method to get my head around. I spent most of my time resisting the urge to paint the forms in a lot more pronounced than an ambient occlusion pass would have them. Much of the form often has its values squashed and the transitions are really smooth, making any errant brushwork spoil the effect of the 3D-ness. It has taught me to really think about where light doesn't get into when you have forms meeting though and to really use soft transitions in low or ambient lighting pieces. It's also a great value check, as if you don't get your values right the colour layer just doesn't work.