Took a class on "dynamic sketching" that helped me to break forms down into simple 3D shapes. The first part of the class relied a lot on getting good at quick perspective. No straight edges, drawn in pen, and don't bother setting up a whole grid (this is for sketching not drafting). In order to help visualize this I wrote a little script here that makes a game about drawing cubes in 2-point perspective when all you can see are other cubes (no horizon even). To play: click on the red node to start drawing. The foremost edge has been drawn for you (setting the position and height of the cube). Draw your first top edge to the left or right and then keep going until the cube is drawn (cubes that straddle the horizon don't require top/bottom sides). Feel free to hit the 'g' key to reset if you want to change the vanishing points or clear the board.