I spend most of my sketchbook time drawing heads and faces. This is partly because faces are what I enjoy drawing the most and partly because I find drawing faces well to be difficult. I feel a constant need to practice drawing the head and face. When I have time and opportunity I enjoy drawing from life. This can happen either in a formal drawing session or by just getting out on the streets with my sketchbook.
I think there is also great value in returning to my library of "How to Draw" books. Here's an example of how I sometimes do this. The drawing above is my copy of a drawing from Burne Hogarth's Drawing the Human Head. It is a simple structural drawing of the head in a 3/4 view from above. This is not a point of view that I use frequently so I thought it would be good to practice. After copying the basic structural drawing a few times I put the book away and do some drawings of my own using this point of view. Below are a few of the drawings I did during this session.
As you can see I'm using the same basic framework and just changing the details. Give it a try, just try to make each face a little different some how. It's a good way to practice.