Artistic Insight: Flip It!
When I started Foam I was having trouble with facial proportions, so guess what I did? I flipped it upside down.
Flipping a painting upside down (or mirroring it left to right, either with an actual mirror or digitally) helps the brain forget what it knows to better register what is actually there in its elements and composition. Basically you're forced to look at the art with a fresh perspective that helps you find more details (and mistakes!) that you might've missed with the right-side-up view. It can also technically stretch the art - you're throwing in "backwards" brushstrokes or marks this way that can refresh your piece.