|Enhanced with Photoshop Smart Sharpen.|
Unsharp mask, despite its name, is usually the best choice for sharpening. There are three controls to be set when using unsharp mask (USM): Radius, threshold, amount. I usually use a radius of about .8 pixels, a threshold level of about 3 and for amount I like to vary the percent but 100% is a good starting point. The radius controls the size of the edges affected (too much and you will not sharpen small details); The threshold controls the brightness level at which sharpening starts (too low a setting and you sharpen grain); and the amount is the overall strength of the sharpening effect (too much and you will produce the infamous halo effect known as over sharpening.)
A couple of caveats: Sharpening is irreversible. Always save an unsharpened original. (I always save my original, unenhanced images. I save my enhanced images under a modified name and thus do not overwrite my original image.) And always apply USM last. Sharpening is the very last thing you do to an image before saving it. Remember, digital images are inherently a wee bit soft; A wee bit of USM before saving is always a good idea.
If you have Photoshop, I have CS5, you might play with the Smart Sharpen setting. I have been quite impressed with it thus far. Click on the Smart Sharpen link to see the Adobe instructions. USM emulates an method used in the good old days of film to give the illusion of a sharper image. Smart Sharpen takes sharpening control to another level.
For more details on USM sharpening, click on this link to the Guide to Image Sharpening. Read the info in the guide and you will know more than the average pro photographer about USM.