After taking some time to figure out what I was drawing and why my sketches improved. I figured I'd try a nice portrait! Well, it was going just fine! I'd learned a few new paint mixing techniques, I was thinking about light and shadow a lot more than usual as I was thinking of my pictures as three dimentional.. Then?
I went to draw the face. It looked like someone had shotgunned clown makeup on a raggedy anne doll. Not cool. So I went back to my sketchbook and started with eyes and lips... then noses... and finally faces.
Obviously I noticed a few flaws but I was able to zero in on WHY a sketch wasn't working! "She has no chin... her lips are bigger than her nose.. there's no smooth curve. More or less it came down to, "That's not how a human looks."
Another bit I picked up on while drawing faces is: When I started... I had no idea what the finished product would look like. I just draw features and hoped for the best. So I stopped... took a breath... and considered things like the shape of the face, the style and mood of the eyes, button nose, upturned nose, rounded noses, thin lips, thick lips, etc. I was able to really draw faster having a goal in mind and the parts came together. Tiny nose and huge lips looks like a clown... huge nose and huge lips looks ugly...
So anyway... don't just sketch. Think about what you're drawing and learn to draw not just the features but also the features as compliments to each other.