On my recent trip I was introduced to a greyscale and value finder. This is used so that you can get your values right in the piece you are doing. A lot of times something is too light or two dark and you just can not see which one is it. With this piece of paper you can solve that problem. Even with colour pictures you can still determine the value using a grey block. I created a PDF for you to download.
Just print it out, cut it out, fold it and glue the sides together. I even went a step further and laminated it and then punched the holes.