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All I was thinking in this demo at my teaching class, The California Art Institute, was to take something fairly complex, the human form, wrapped in a luscious robe, and make it simple, stupid.
Decide on a focal point and stick to it. This is also really important in landscape painting on site when the light is moving fast.
Deciding on the focal point can be obvious sometimes - or not. Tons of books have been written on the subject. Edgar Payne a favorite of mine for learning more about compositional concepts. http://www.edgarpayne.com/
"Composition of Outdoor Painting" By Edgar Payne
I tend to trust my instincts and do what excites me, for better or worse!