Don't draw heads, construct them! On your first page of heads you've got a couple that are just made from planes, and so have been constructed as 3D heads. They are the best ones! The proportions and shapes look correct, plus there's some form to them. Start with something like that before adding the details, get your form right first. Also try using it in conjunction with references. Look at the references to get an idea of where the plains are. This will get you used to how everything sits on a head - at the moment your eyes are too close together and aren't wide enough.
I like the thumbnail postures you've done, I'd like to see more of them.
Keep working away.