How to draw a head from all angles from imagination?
I've got some manga art books that are helpful - they give guidelines for drawing heads/bodies at all sorts of angles, whether by using skeleton frameworks, or just drawing freehand. 

Also, if you're happy taking a sketchbook around with you, try doing 20-second speed sketches of real people (using pencil, biro, or whatever medium you prefer). Real people don't tend to stand still that long, so you'll have to be quick, but it's a great way of practising natural head positions. (Places like shopping centres or looking out of café windows can be helpful - some places put you below head level, and other put you a storey or two above where people walk).