here look at this-
http://www.deviantart.com/view/21910338/
The light source is in the back-- that doesnt mean none of the things in front have no highlights- the light will "roll around" a but and highlight around the object. Also the face isnt flat- so that bit of white under the eye on the cheek is also getting hit by some light- other spots like that are being hit by light that is reflected (also why that whole side isnt really dark-
As for the clothing- you have to study it yourself- remember- at joints and seams theres going to be some folding cloth-- folded areas of cloth often picks up a bit of a highlight (notice the highlights on the folds on his arms- they follow the wrinkles and folds- while under them its darker.
If the clothing is TIGHT- the shadows will move with the curves in the anatomy- if its loose there will be lots of baggy wrinkles and thus lots of shading and highlights- if they are wearing rather straight clothing - there has to be wrinkles somewhere-- make sure you draw them- notice how clothing falls and gets pushed around- there will be at least a couple of wrinkles or folds- if not then your a paper doll!
Heres another picture by the same artist with a light source coming from the left front.
http://www.deviantart.com/view/22375741/
Hope this helps