I would say that the eyes are the most important part of sketching a face and that the sketch should begin with the eyes. From there, once I have at least an outline of the eyes, using the eyes as a reference point, I sketch the nose, then the lips using the eyes and the nose as a reference point.
After the eyes, nose and lips are sketched, I proceed with the perimeter of the face then the hair. The hair is tricky, especially long hair. Drawing long hair takes long quick strokes with no hesitations.
Once I have the basic outline of the face and all its features, I begin sketching in the details, adding shades with the pencil. Learn to work with the tip of the pencil, using flat sides as well as sharp edges.
When sketching a face, begin with the eyes, then nose, then lips, then work your sketch around them. It usually takes me around 1 1/2 hours to complete the sketch of a face on a 9X12 sheet of drawing paper.
@ mattjanacone I totally agree with you that the most important part of sketching a face is begin with the eyes. I think this tutorial explains it really well: http://www.jerrysartarama.com/art-lessons/Artists/Adam-Hughes/Adam-Hughes-How-to-Draw-Eyes.html