This subject seems to be a big mystery to a lot of Photoshop users so here are a few suggestions:

1- Never use the sharpen function in your digital camera or your scanner. Do it in Photoshop - as follows.

2- Use unsharp mask as the preferred method for sharpening your images - rather than sharpen (more on sharpen later).

3- Never sharpen your original image. Keep it original and sharpen the printer or web version AFTER it has been appropiately resized for the application. This is very important. However, there is one exception. You might want to first remove any "digital fog" created by AA filters by applying a very gentle USM of .4 pixel width, 150%, and 0 levels.

4- For fine print work, consider sharpening only the channel that has the least detail (windows, show channels, split channels). This works on some images, especially if one channel has very little information.

5- The BEST way to use unsharp mask, however, is using luminosity. Make a duplicate layer. On the top layer change the "normal" to "luminosity". Use unsharp mask and adjust to taste. 

6- Now a few general starting points. Your taste will vary. On large 60-100 meg images I would suggest starting with "amount" 150%, "radius" between 1.5 - 4, and "threshold" at 0-4. This would work for medium size prints. Then use your "opacity" slider to fine tune the opacity. I would try 70% opacity. On images around 30 meg, I would try a radius between 1.4 to 3 with all the other parameters staying the same.

7- The Lab method. Under Image - Mode - select Lab color. Now open Channels. (Windows - Channels). Now select (highlighted in blue) the Lightness channel. Use USM at .3, 200%, and 0 Threshold. You are only sharpening the Lightness channel! Many prefer this method over #5. I love it. Try them both.

8- Lastly. Consider sharpening different areas of your photograph to different degrees. Example: Don't sharpen the sky. Use medium sharpness on far away detail and more sharpening with close detail.