Hi, edd! Welcome to the forum.
I'd like to suggest a few areas for you to study.
Since you seem to be painting a lot of faces, check out the Asaro head. Do some drawings/paintings of it and try to memorize it. Having it in your head, at least to some extent, will help with all your heads and faces down the road.
Think of things as forms, planes, occupying space. Check out drawabox.com to see what I mean and some exercises you might go through later.
Think about how the light and surface of things interact, as in the fall off of light, where the light is blocked. In general, learn all the lighting factors and use them in your images. Read the three pages here
http://www.huevaluechroma.com/021.php
Learn to measure and copy from a reference really well. You might be able to do this already, but I think I didn't see that above. It's a necessary evil to be able to copy, even if it's not the end goal we have as artists.