In my voracious (and distracting-from-work) reading about all things sketchy and brushy lately, I caught what sounds like a cool hint:
Draw something upside down.
No, don't flip over, pencil in hand.
Turn a photo or drawing you want to sketch upside down, and draw what you see. The idea is that you will see the forms that are really there -- the way you used to see as a child, before logic muddied things up. It's drawing without assuming the forms add up to anything you've learned to recognize.
I think I will try this this weekend.