Other than walking away from it or starting over which are sometimes the ultimate solution, here is my trick.
Take a quick snapshot on your phone.
This is a beautifully edited picture of my finished project to show you how it all ended up.
Let me now walk though my snapshot.
Wonderfully candid, un-Photo-shopped pictures of my work space.
I wanted to incorporate the dots throughout the piece. I added that large black and white print on the right and wasn't quite sure it was the right way to go.
It turned out to be WAY too much. Either the print size or the harsh color was an immediate visceral reaction AFTER taking the picture. I would have been disappointed if I had left it there.
This also works well when you get to the detail stage of your composition. It you feel like it 'needs something' snap a picture.
You will almost immediately know what needs to be added. If you can't figure it out even after that resort to plan B - walk away. Hopefully you won't have to resort to plan C (starting over) or the dreaded plan D which is too terrible to even think about.