If you are a realist painter:

--Do not compose your works as though you were composing photographs.

--Do not use photographic techniques on your paintings (cropping, defocusing, disproportioning limbs due to lens distortion, details running out of frame, and the like)

--Paint the background first, especially if it is architectural. Then paint the figure or figures next, for the figures will adjust to the background, but not the other way around.

--When painting portraits, do not make the background a hodgepodge of excess paint. Either paint a concrete background with appropriate details or use a completely new color mixture altogether. The background of a portrait should never be an afterthought. And remember that the background should respond to lighting, as the human figure does.