The biggest problem most painters face is value. More than anything else, poor value keeps their paintings from having a sense of light. One way to train your eyes to see the whole image and therefore the value relationship of light to dark is to start by dividing the image into light and shadow, one value for each group.
This posterized version will not only simplify your work but forces a decision about which elements belong in light and which belong in shadow. Once established you have a template to guide your color choices for the rest of the painting. Anything in the shadow must read as shadow from then on and anything in the light must read as light. Keep reflected lights as part of the shadows, reflections must read as whatever they are reflecting.
Whatever details get added they should never obliterate the initial poster shapes. This is the tricky part because as people add details and describe forms they lose track of their values and the big divisions of light and shadow sacrificing them for more information. Instead they should describe the elements within those values set in the beginning.