Some time ago I got question about how I approach portrait and face drawing. I posted about it on Tumblr, but I thought why now share my thoughts here on DA also
The first thing if You want to do portraits is to know anatomy. Start with learning what is under the skin and understand how it affects the face. When You got that covered You must know how face is structured. That will give You the knowledge how to light head. Head is a cube-like object. (yea! NOT sphere!) On face we have other little cube-like object (nose).
A great exercise is to find photos of a face and draw structure lines over it.
You see clearly now? The structure of a nose, where cheeks are, forehead etc.
After doing this You will be able to create the face from a memory using these helping lines.
Like here. This is Michael Hampton exercise:
You see how everything fits thanks to the structure lines?
Drawing a face requires TONS of practice. Find thousand photo references and practice.