These days I try to spare sometime outside work to practice on faces, headshots or bust, in greyscale/black and white.
Sometime I use photos as reference, sometime just trust wherever the brushstroke and the imagination take me.
Hope by doing this I'll be more aware on values, how to simplify shape more, and anatomy (I'm really sucks at creating good angle on faces).
For more frequent update on this, you can find it on my instagram. Have a nice day to you!