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.
one of the instructors in our Illustration program made everyone draw from life (models) with their left hand for several days. It was painful (and frustrating) for the students to readjust their thinking but after doing it, all said it was a breakthrough in how they saw things.