Once you feel satisfied with a piece, take a step back, and remove it from your thoughts. After several hours, or even days, of separation, return to it and begin your revisions.
It's so easy to get caught up in your work and become jaded by it, unknowingly glancing over major mistakes and missing great opportunities for improvement. As artists, we need to realize when enough is enough. Take a break every once in a while. Your work will improve exponentially as a result.
We all know the mirror or flip-canvas-horizontal trick (If not, I'll surely cover that in another post). This is just another version of that. It allows you to see your image in a whole new light, as though you have a second, fresh pair of eyes reviewing your piece.
Trust me, I have spent many days working unto the wee hours of the morning only to get frustrated at myself because I can't figure out why I can't get the darn image to work. It's not until the next day, with rested eyes and a refurbished mind, that I'm able to clearly see every single stupid mistake that I made the night before. It's not worth the headache. Save yourself some time, frustration, and energy, and step away for a moment or two.