Try out drawing real life stuff. That is the most important thing one can ever suggest - gaining expirience in drawing real stuff. First of all, try something geometricaly simple and prefferably of white color, so that you can easily see the shadow. A paper cube, pyramind, a marble (sphere) - EVERYTHING can be simplified as a construction made out of such figures, and when you understand how these objects appear to the eye, you can simplify and understand the form of nearly any other object. For example, you see a head, but start off with drawing a sphere, instead of a contour. With the help of this simplification, you can procede to create a 3 dimensional form, drawing the head as a whole, not seperate pieces of flat contour. 

Combining the understanding of form with raw experience, you soon will be able to draw what you want to, as it is understanding you need to create art and not just copy real life. If you just draw what you see, you wont be able to draw what you imagine.