here is another workflow-post.
1. reduce the object to its simplest shape
2. fill it with the midtone
3. brush in the shadow. I use a soft round brush for this.
4. paint in the light
5. paint in the reflected light (it is always darker than the direct light, ecxept when there is a really reflective material reflecting it like chrome for example)
6. use the smudge tool on a seperate layer with "sample all layers" turned on and get a first pass of structure in. The good thing about the smudge tool is, that it keeps the light-to-dark pattern. I just took a soft round brush, set opacity on pen pressure and switched on scatter on both axes
7. take whatever brush you like and paint in details on another layer
8. blur the outer edge a little bit everywhere, except in the focal area
Have fun :)