For a 640-800 pixel wide web image try a Radius of 0.6, a threshold of 0, and then adjust Amount until it looks OK. (Don't be surprised if it''s a large number, just move it until it looks OK.) Once it looks OK, try moving the Radius around a bit - anywhere from 0.3 to 0.8 depending on the image. Usually the finer the details (fur, feathers) the lower the number, but it's all a matter of taste and the specifics of the image. If this is a high ISO image try moving the Threshold to somehere around 1-3. The Amount will have to go up even more once the Threshold goes up.
For a print image, zoom to 50% and choose a Radius of somewhere between 1/100th and 1/200th of the print DPI. So for 300 DPI (which is very likely) choose somewhere between 1.5 and 3.0. Again Threshold 0 unless it's high ISO, and play with the amount until it looks good.
Watch out for Sharpening Artifacts which are usually halos around high-contrast lines and small spots which are too white. It's a very good idea to flip between the unsharpened and sharpened image to spot these things. If you have USM open just enable & disable preview. If you've already applied the sharpening just hit Ctrl-Z to flip back and forth.
If you're printing for the first time it's a good idea to try 3 different sharpening strengths and compare. So if my image looked good with an Amount of 250% then I'd also do one at 200% and another at 300% and then compare the 3. If you're looking to print a large print for the first time then it's a real good idea to crop out a 4x6 of an important section and try 3 different sharpen strengths and print the 3 4x6s for much less than the price of a 20x30. (And if you do this, try 3 different saturation levels too.)
Remember that sharpening is very subjective - it's difficult to be wrong, since it's what *you* like.
Also note that viewing distance plays a big part, too. A 4x6 that people will hold in their hand will look very different from a 20x30 which people will see from 6 or 8 feet away.