As a kid I discovered everything looks completely different upside down. Later I learned, when drawing from a picture, turning things upside down is a good way to check your accuracy.
It's also good practice to draw with your reference upside down. Shapes that are all too familiar become unfamiliar and you are forced to draw what your eyes see, not what your "left brain" tells you you see.
Check out Drawing on the Right Side of the Brain by Betty Edwards, p. 55.