You have a nice handle on airbrushing, but I'd suggest you sharpen your edges up, so that they're not so blurry. It'll help to pop your images out and give them better dimension.
As for the shading, don't be afraid to go darker, it will help to show better form. To explain shadows in a nutshell, cast shadows always have a harder edge, and gradually get less sharp the farther ways they are from the object casting the shadow.
The other main type of shadow, is the core. Core shadows always happen whenever you shift planes into perspective, and every object can be broken down into planes.
As you can see in the image below, where ever there's a plane break(for example, where the front cheek plane hits the edge of the side cheek plane), there's almost a darker "line" of shadow.
Hope this helps!