Shading 101

On the left is DO, the right is DON'T
Working with fabric
Blue: DON'T shade straight from left to right. A leg is not a flat box, so the lighting never hits it like that. Tight fabric won't be any different. And loose fabric drapes over legs and has a lot of folds as a result. By folds, I mean lines going across the model that borders different parts of the leg, arm, chest, etc.
Red: Despite what cartoons will show, there are no darker lines around the edges of objects or people. The only thing that separates one thing from another is lighting. so DO add a lot of different hues on the border to show where the light is mainly coming from.
Yellow: The only problem with the right is that the model is fabric, not skin. As I said, fabric is just a layer, not a surface, so DON'T shade clothes as a solid thing.
Tan: And here's why to NOT always treat skin as fabric. Skin is soft, but unless you're doing wrinkles, it covers our skeleton in round shapes. So DO rounded shading on skin.