One of the important tricks I teach painters is to turn their paintings upside down on the easel after they have laid down a rough sketch or underpainting. It removes the "meaning" from the objects in the painting and allows them to see the relationship between dark and light, positive and negative space much more easily. If the composition doesn't work upside down, it's probably not very good.
i had painting teachers like that.
it is one of the best techniques there is ...