Sharpening an image

Once you have a CMYK image at the size you’ll be using,  you can apply a little sharpening to punch it up. Make sure you are looking at your image at 100% (View menu / Actual Pixels). Go to Filter > Sharpen  (the flyout gives you several pre-set sharpening options as well as Smart Sharpen in CS2 and CS3) and select Unsharp Mask. Unsharp Mask enhances the edges of an image by making the dark edge darker and the light edge lighter.
If you have, for example, an image of a football on the sidewalk, the filter will make the sidewalk lighter where it touches the football and the outside edge of the football will get darker. The Amount slider controls how much lighter or darker the edges will get. Increasing the amount too much will produce the "halo" effect which is seen around the candles on the over-sharpened Birthday Cake.

The Radius slider controls how large an area the effect covers. The larger the radius the less the sharpening impact visually. In most cases, you'll want to keep it under 2 pixels. The Threshold slider controls the size of the objects to be acted upon. If you see that you are sharpening noise or specs of dust in an image, increase the Threshold.To start with I will generally turn up the Amount higher than needed to better see the effect of moves I make with the Radius and Threshold sliders. Next I'll adjust the Radius to what looks good for this particular image. Lots of edges close together, grass, hair etc, will need a smaller Radius setting than would images that contain large objects. The next step is to adjust the Threshold to sharpen only the image and not enhance the grain of the film or the pores on a person's face. Finally go back to Amount and bring it down to a level that looks good and click OK. The amount of sharpening an image needs depends on the quality and contrast of the image to begin with but any scanned images and most digital images are improved with a little sharpening.

The rose on the left was sharpened 50% while the one on the right was sharpened 150%, Radius and Threshold set to 1 for both. You can see that by increasing the Amount you bring out the veins in the petals but at the same time accentuate the halo effect around the petal edges. A light hand is generally needed here - you are enhancing the image, not applying a special effect.
For people pictures, a lot of times I will sharpen only the Black channel. This will allow you to sharpen hair and glasses quite a bit without accentuating colored clothing too much. Use caution as this technique will often enhance "five-o-clock shadow" on a man's face.
Remember, once you close the image, the sharpening cannot be undone. You should always work on a copy of your original image as different sizes may require different amounts of sharpening.