After reading the Greg Albert book on composition I cringe at the paintings I post knowing that they could have been so much better with a little work on the composition. To help me plan paintings I keep a notebook where I do a number of thumbnails before coming up with a satifying image. It's at this stage that it becomes obvious how important it is to really know my subject. For instance if I'm working on a landscape I choose only scenes of places I visit often so I can imagine myself on site. This allows me to use photos just as inspirations and not depend on the photo for the arrangement of elements.