Faces and Features

I've been working a lot on just faces, recently.  The reason, of course, being that this is one of the hardest things for an aspiring artist to get right.  Facial features aren't as forgiving as static objects; if anything is out of place or proportion, things just look not quite right. Here are a few of my better pieces, but I still have a long way to go:

 

  

That last one is a self-portrait based on a picture my wife took.  As you can see, you can tell each one of these is a face without a problem, but I feel like my shading is what's really deficient.  Anyone with advice on how to really make these faces come to life, I'd love to hear it in the comments.