More shading

I have been playing around with shading on the Evil-genius pic I posted a few days ago. I don't think the shadows are exactly perfect, but I like the effect of lots of little shadow detailing. It was mostly done by hand, though I did copy the shapes of the robot, tree and zeppelin and then free-rotate/transform them, then make them into masks to adjust brightness. The shadow stuff on the main character was all done freehand (well, freemouse anyway).

I think I like it as a technique, and I'm a little curious about ways of adding shading like that in easily - by drawing in guidelines on the original image in another colour, for example. I tend to find pen-and-paper drawing quite quick these days, as I mostly operate within my comfort zone. Adding shading lines will be an interesting challenge.

OTOH, drawing all the elements separately (eg main character, robot, background etc) would probably allow for some very fast drop shadow creation using existing tools in the paint software. It's not the way I currently operate, but I could play around with it as an idea...