First off i lay no claims to this concept..but i have witness alot of what i would consider the top drawers on this site using a form of this concept.This does not only apply to traditional forms of art such as graphite, color pencils,painting but applies to just about all of the mediums and includes photography.
Shading:For realistic drawing it is a must,but if applied wrong this can make your drawing flat , with no depth.done right you will achive a level of drawing or painting etc which will draw the viewer in with no effort.
Study: Most of us use a Ref photo, study it before you start drawing it,plan your drawing out
and keep to it,,draw light and build values once you have abtained a correct line drawing.
Material i use are Bristol Vellum..4h,2h,hb,2b,4b pencils , i begin by finding my darks and looking for shapes in the values..i then apply the 4h working feathering it out from my darkest value to the lightest,once ive obtained the shape i apply the 2h over the top and
feather it out over the 4h..being careful not to go beyond where i layed the 4h at..once ive accomplished this i go back to the 4h to blend the 2h in and widden the feather of the 4h..i continue this with each of the pencils above until i have matched the value of the area im applying tone in..really most drawing is nothing more then matching values..keep in mind that," what goes away from you goes into the dark" and "what comes at you is in the light"..look for the rounds in your ref photo anatomy is very important in portrait drawing to obtain depth and a 3d look..brushing off the area you are working on will remove excess graphite and allow you to apply more..lightly apply your layers and build values gradually..dont rush it or youll find out about the infamous tear in the paper.# values are a must in your drawings..dark..mid-tone..light the impact of the drawing will be greater on the viewer if you accomplish this...okay thats all for now...if you have question please feel free to contact me..ref url:5pencils.com or look up Darrel Tank..ill post a sphere..you can also see demos of this concept on you-tube...i am not an advocate of his , i simply like the way he draws.. on his site he has free demo's of this concept and he freely gives them away