Incorrect sharpening ruins more images than any other thing.

Before starting any work on an image make sure any sharpening tools in any programmes or filters to be used to produce the final image are set to their minimum or, if possible, disabled.

After any work that is considered necessary the 300ppi tiff files are stored unsharpened to CD or DVD. This means a file can be resized or altered to suit the printing requirements without having to worry if any sharpening has taken place.

No matter what method of sharpening you use this should be the last process before printing, processing for commercial print or saving for use on the web.

A good method of sharpening is the Lab or Lightness channel method.

Image>mode>lab colour

Channels> select lightness channel. Do not delete the other channels.

Apply sharpening, I use USM at 100% Radius 1 Threshold 2

Image>mode>RGB

The perceived advantage of this method is you are applying sharpening to the data channel only. As you are not using the colour channels you reduce the possibility of colour sharpening artifacts to virtually zero.

An other good method of sharpening that does not affect the colour channels is the Luminosity method, this is now my standard.

Open an image and apply a USM

Immediately open the Edit menu and select Fade USM. In the popup menu change the mode fro normal to luminosity. Click OK and the sharpening is applied to the luminosity channel only.

It is possible to apply the USM at least twice using either of these methods without inducing any artifacts.