Positive and Negative Shapes in the Face

Drawing a good likeness is tough.  A portrait could easily fall apart if one feature is slightly larger or askew from another. Use the negative and positive shapes in between features to help you gauge these subtle distances. William Maughan uses this rule of thumb to help him place shadows on the face, I think it could be universally used to help place the eyes in relation to the hairline for example.

This image is from The Artist's Complete Guide to Drawing the Head by William Maughan - a recommended book!

These images are from Drawing Fashion by Bill Thames. Each of the pages has generic proportions of an adult female and male face as well as step-by-step instruction on how to draw a face without reference. Its also helpful to see how the features of the face (eyes, nose, mouth, chin) can be easily turned and drawn with consistency by using guidelines based on proportions.

Ear Reference and Practice

Click to enlarge.

from Face Parts by Simon Jennings

Profile Proportions of the Face

Children look like children because the lower half of their face has not grown to adult proportions.  In the above image note how the horizontal midline of the face is lower in that of the child.  That shift also affects the placement of the ears.

from Face Parts by Simon Jennings

Facial Wrinkles

Although this is a guide for facial surgeons, it is also an excellent reference for artists drawing wrinkles.

I didn't write down which site I pulled this image from.  I'll add credit when I do find the link.

Rubens Portraits

Rubens' linework is so lively, copying his drawings is great practice. ;)
via

Movement of the Jaw

Just as the image illustrates, the jaw does not open and pivot at the corners of the mouth, but at a point behind the teeth on the upper jaw.

from Human Anatomy by Jose M. Parramon

Muscles of the Face

from Human Anatomy by Jose M. Parramon