The typical way most digital photographers use Photoshop to sharpen images is by using the "unsharp mask" filter. Remember, sharpening should always be the last and final step before your print or screen output. The image size should be finalized and all layers flattened.
The typical steps used to sharpen are:
The results are reasonably good but when the image is examined closely there are usually halos around edges and color artifacts. The image ends up looking unnatural.
The problem with using unsharp mask in this manner is that the filter is being applied to all the channels - RGB. The sharpening is not really required for color channels.
Recommendation:
First lets examine the three controls in the unsharp mask filter. what they mean and do.
Amount - this is measured in % and defines how much sharpening is applied to the image with respect to the Radius and Threshold values.
Radius - This determines the number of pixels from the edge that the sharpening will be applied to
Threshold - This determines how different one pixel is to its neighbor for it to be considered an edge and thereby be applicable for sharpening. Hence the lower the number the more intense the sharpening.
If the color channels do not need sharpening lets covert the image to "Image - Mode - LAB Color". (Here the L channel carries the luminescence values and the A and B channels carry all the color information)