One of the most common mistakes when it comes to editing images is not taking into account the brightness of your computer screen.
I know it's a common mistake for the simple fact that I was doing it for years, and have noticed many images that look great, but are just a bit on the dark side
A very simple fix to this issue is to lower the brightness of your computer screen.
Personally, I keep the brightness up all the way when I'm doing the fine editing, and when I'm going to do the final brightness balance of the final image, I will turn the brightness down 3-4 points.
The reason for this is twofold. First and foremost being if you want to get your image printed, it will (and I stress it WILL) be darker than you anticipated. I've had images that looked perfectly fine on screen, set them to the printer and when they came back, much of the detail was hidden in shadow. Secondly, people have different settings on their screens. They won't get the whole range of detail if it's too dark. Plus if you're like me and enter contests and submit to magazines etc, they could be rejected solely on the brightness despite being of extraordinary content.
So there you have it. A simple solution to a common mistake when editing images! I've included a couple examples below. The image on the left was edited and looks great on the screen, but they are actually too dark when they get printed!