Ambient Occlusion

This picture looks like it's shaded correctly, but if you look carefully you'll realize that the light isn't coming from any particular direction. "Ambient occlusion" is a fairly new way to do shading in computer graphics to get results like this. It basically simulates light coming equally from all directions, like on a cloudy day. This is more realistic than you would think, since a surprising amount of light comes from bounces off other surfaces in the scene, rather than your direct primary light source. But you don't need to simulate all those bounces-- that's called radiosity and it takes a lot of computation. Instead, just look at the angle each patch of the scene forms with nearby patches. An area that is in a valley (like under the eaves of the house) will tend to be shadowed.
You can get this same effect when painting a minature. When you're done painting it, give the whole thing a coat of thinned black paint. Then pat off the paint. Some paint will get stuck in the wrinkles and not come off when you pat it.