I've always been told that I should train my brain to see something and then be able to break it down into basic 3D shapes. Currently, however, I'm not sure I have this concept down, and I want to use the most efficient method as possible to practice this.
So I'm asking all the artists who feel like they understand this aspect of drawing: What sort of practice did you use to cement it in your mind, and about how long did it take until you felt you had it down?