What are some key considerations when approaching shading on a realistic rendering

The advice I will give you is to use it wisely to bring the viewer's eye onto your subject and, if you are drawing a living being, enhance certain traits to make it livelier; give it some emotion.

Use the shading to bring the eye onto your subject, enhancing traits to make it liveier, give it life. The first thing the eye will see is the lightest part, and the darkest part. Make certain these are done well, to bring the eye to the subject.

Look at this "computer" rendering using shading. It does both of these well.

http://www.mpi-inf.mpg.de/~guenther/carpaint/img/teaser1.png