Sharpening

There are several different methods to sharpen an image. I recommend Smart Sharpen. You can sharpen certain areas of an image during retouching if you make a mask (like the eyes, or the hair, or the flower, or whatever you want to be the center of attention). Before you print, it's a good idea to sharpen the whole image just a little ("output sharpening"), as making a print can reduce the sharpness of an image just a bit. But be careful not to oversharpen!

*Remember that all sharpening needs to be done on a new merged layer on top of the layers stack, ideally after you are done with all other retouching. To make this new merged layer, go to Select > All, Edit > Copy Merged,  Edit > Paste. (Or the shortcut is Command/Shift/Option/E all at the same time.)

Smart Sharpen: 

1. First, make a background copy or make a new merged layer on top of the stack (see above).

2. Go to FILTER > SHARPEN > SMART SHARPEN. Depending on your file, you will likely want an "amount" of 100 to 200, and a "radius" of 1-3. 

If you want to do selective sharpening, you can make a mask, invert the mask (command i) and then paint with a soft white brush on the areas you want to be sharpened.

Be sure to save as a .TIFF or .PSD file to maintain your layers.