In your mind try and break the figure down into three dimensional shapes, instead of two dimensional shapes. Try to visualize how your subject would look like as a mannequin. You'll begin to see how one shape would lay in front of another, or how two shapes connect. Most importantly, you'll begin to visualize how you can draw your figure and make it look "real" because it will look 3-D. In the early stages of your figure drawing you could flesh out the stick figure using these types of shapes. Then when "Thinking in 3-D" becomes natural to you, you can go back to drawing simpler shapes, knowing what the 3-D shape would look like in your head.