Well , I will give it another shot ! Hopefully I will be more helpful this time.
Really , it depends on the shape of the object in question. What you will want to visualise is the light interacting with the object. When the light hits something, it will then bounce away (for the most part). What you are probably going to want to look out for is, "Where is a place that the light won't get to?"
For example, let's say that you have a person's shoulder. The light source is behind them, maybe around head level, like a lamp or a low setting sun. Now let's take the whole scene and rotate it so that you are looking straight at the shoulder (like a profile view of a person). So , the light source is off to one side. The light coming in toward the person's back will hit it and bounce off, so you will either use no shading, or perhaps a little bit of highlighting. On the other hand, once you go over the crest of the shoulder and start going down the arm/chest, the light isn't going to be able to reach that because their arm/shoulder/back is in the way ! So you would shade that region. You can either use the cel-style in which you basically only have one level of shading, or the realistic style that is totally dynamic; the further you get from the edge of the light (ie, going from the top of the shoulder down to the chest), the darker it gets. I recently have used a combination of the two.
As for the shape, it also depends on the object. A rounded object would likely have a shadow that follows the rounded edge. So if you have, say, a leg ... since a leg is rounded, your shading line would also be rounded. This shows the "depth" of the leg, and lets the viewer KNOW that it is rounded; if it just goes straight, sometimes it can make the leg look blocky.
Ahh! I don't know if that helped anyway. Someone else give it a go!
~Syn