Like three years ago when I took a painting class with Brooks Anderson, one of the big things he espouses is underpainting. This year he encouraged even more laters than last time. We first covered the canvas with a putty colored acrylic gesso, followed by a medium value red layer of oil. After that you paint an underpainting either in random shapes or block in colors different then the ending colors you plan for a scene. You are to use no white in an underpainting, for then the painting will lose its intensity.
Brooks even goes so far as to claim the the most important part of the painting is its underpainting. Below you can see both some of my underpaintings as well as those of others.