One very effective way used by many artists to add subtle yet effective styles to your artwork as a finishing touch. Here is how to do it in Paint Tool Sai.
I know I need too I just mentioned it because most people have an average monitor I am gonna invest in something with an IPS panel in the near future ;D
But the trick is very handy and I appreciate it, although I don't have Sai. I assume it would be a very similar process in Photoshop although I do not know if the clipping mask is the same.