High Pass Filter

There are many ways to sharpen images in Photoshop, but the High Pass Filter is one of the simplest and easiest to use. I am using Photoshop CS5 on a PC. For demonstration purposes I shall use a rather unflattering photo of myself, but the process works just as well on beautiful photos or complex, layered fx images. Here are the before and after versions:

The effect here is probably a bit too strong, but I thought it best to exaggerate it to make the difference clear.

The process is very simple. First duplicate the image (Ctrl+J). 

If you want to try it on top of a multi-layered fx image without flattening everything, highlight the top layer and then press Shift + Ctrl + Alt + E to create a flattened copy of all the layers on top.

Change the Blend Mode of the new layer to Overlay, which will probably look pretty horrible, but worry not!

Go to Filter > Other > High Pass:

High Pass is very much easier than messing about with Filter > Sharpen and all the alternatives, so give it a try if you have never used it before.