YOUR BEST CRITIC AND ALLY IS YOUR CAMERA!
Take a picture of your painting as soon as you’re done mapping your under-painting. Upload that photo to your computer screen and you’ll be able to see MUCH better any potential flaws in your composition. It really feels like you are looking at someone else’s painting, so you can be more objective about it’s problems in shapes and composition.
Take another shot of it when you’re about ½ way done and again show it in the computer screen. That shot will help you see where VALUES need to be increased or decreased to help the overall composition.
Take yet another shot whenever you’re “stuck” with a 95% finished painting that doesn’t look totally “real”. You might see then color problems or where the shadow values should be increased to make things “pop”.