Draw Up a Chair

There is a drawing exercise from Betty Edwards' Drawing on the Right Side of The Brain that I sometimes use with my humanities students. You turn a chair upside down and then you draw it. Your brain doesn't recognize it in this position and you are more likely to draw what you see than what you think it looks like, hence your perspective and proportion will be more accurate.

There is another exercise in the same book that suggests you draw negative space, the space in between the trees instead of the tree itself. It has a similar effect on your brain, you pay more attention to the lines you see and draw what is really there.