There is no substitute for painting from life. An accomplished pianist will say, "When I miss a practice, I can tell the difference.  When I miss two practices, you can tell the difference."  It has something to do with eye/hand coordination and muscle memory.  That's why it's important to paint regularly from a live model.  At Southgate Studio in The Factory at Franklin, Tennessee, portrait open studio painters meet every Monday to paint from life.  Many local portrait painters have trained their eye and improved their skill through these practice sessions.  Here is a recent four hour sketch.