A Sharper Image

The figure labeled "Unsharp Central" shows the unsharp masking control panel from Photoshop. (Corel PhotoPaint's is quite similar, differing only in that Corel doesn't permit decimal values for the radius.) At top left is a small preview window showing the effect your settings will have on the image. You can scroll this around the picture simply by clicking anywhere on it and dragging in the direction you want to go. You can also zoom in or out by clicking the + or - button below the window, although I find it most useful to work at 100%. The "preview" check-box lets you view the effect of your settings on the entire image. (This can take a while if your image is large.)
The controls themselves appear at the bottom of the window: The Amount slider dictates how strongly the effect should be applied, while the Radius control tells the program how wide an area (in pixels) to scan over when looking for an edge. The area specified by the Radius control also governs how large a region will be affected on each side of edges that are found. The Threshold slider is more subtle: It sets a minimum "edge contrast," below which the unsharp masking operator won't have any effect. In other words, minor variations in contrast will be ignored if below this value. (The Threshold control can be very useful when sharpening portraits: You want the hair and clothes to snap, but don't want to emphasize every tiny blemish on the subject's skin. By playing with the Threshold setting, you can often keep the sharpening from affecting the subject's face, yet still work where you really need it.)
Proper Use
I said at the outset that sharpening can't do much for an out-of-focus original. This is because the blurring in poorly focused photos spreads the picture information out across too wide an area to have any hope of reconstructing it, unless you happen to work for NASA. On the other hand, scanners and particularly printers tend to blur images only slightly during the reproduction process, and in a very controlled fashion. This sort of blurring is relatively easy to compensate for with sharpening, and the results can be impressive, indeed. The ideal amount of sharpening to use is that which just compensates for the effects of the scanning/printing process: Any more will produce artificial, coarse-looking images.

No Sharpening	This file shows a range of detail, making it a good subject for our sharpening test.
Radius 0.5, Amount 100%	Here, the effect of sharpening is just beginning to be felt, although it probably won't be too evident in a printed reproduction.
Radius 1.2, Amount 170%	This will probably look about right in print, although it seemed much too severe on my CRT. Notice how much more defined the veins and texture are in the leaves.
Radius 3.0, Amount 250%	This is clearly too much! Notice how all the fine detail has been lost, and the "halos" around contrasting objects. This is particularly evident in the siding on the house in the background: Note the visible white line immediately above the shadow under each clapboard. If you see false "details" like these around the edges of your objects, you've overdone it: You probably need to reduce the amount and radius both.

The first and most critical step is to decide what radius setting to use. Too small, and there will be little or no effect. Too large, and details will be obscured by the sharpening process itself. (In fact, you can get some pretty wild creative effects by deliberately using very large radius values with the unsharp masking filter.) I generally decide on the best radius to use by cranking the Amount control all the way up, and then adjusting the Radius slider until I just start to see a thickening of fine detail and "halos" around objects. Leaving the Radius slider at that setting, I then pull the Amount control down to something reasonable, usually around 150%.
An important note in this though: Since you're concerned about creating a sharp print, the printed output is where you need to look to decide how much sharpening is appropriate. Make several copies of the same image, and try different sharpening variations on each. Then, print them all and see which looks best. With low-end (i.e., non-photo-quality) inkjet printers, you'll probably find that what looks good on paper looks like way too much on the computer screen. Part of the skill in applying sharpening lies in knowing how to translate what you're seeing on the screen to what will appear on paper.
As an aid to understanding the effect of the different controls, we've cropped a section of detail from an image, and reproduced it with here several different radius and amount settings.
Go Forth and Sharpen!
Moderate use of the sharpening tools in image-editing software can dramatically improve your printed output, regardless of the printer you're using. Use the tools appropriately and subtly, and all people will notice is how great your pictures look!