I don't use USM very much. I use high pass sharpening for most images. It's much better to avoid halos etc.
M
Think you can avoid halos pretty well by using small radius and threshold.
About what settings work well it totaly depends on the image size and whats on it. How USM works and how it should be used is explaned in most books dealing with PS or other photoediting programs and also you should be able to find some good advices by Goggling up "unsharpen mask".
Then of course this a matter of experimenting and find your own preference. I would start by using small radius (f. inst. 0,2-5,0) and small threshold (0 - radius value) and then vary the amount untill you get the results that satisfies you. Good luck (and patience).
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I usually use settings a little lower than others have cited. I find oversharpened, haloed images more bothersome than a little softness. I also typically prefer to apply 2 "lighter" passes with USM than one with higher settings in order to provde more control over the process.
It is correct that the amount of sharpening needs to be less on a smaller image. That's because each element is made up of fewer pixels. A rope which is 8 pixels thick on a print image might only be 2 pixels wide on a web image; apply a 1.5 pixel sharpening effect and you are affecting the whole element, not just the edges.
I have some examples of sharpening (and over sharpening) in this pBase gallery.
My average settings for DPC-sized images are
66-88% * 0.6-0.8 dia * 5-7 threshold
You should always at least check for needed sharpening after re-sizing, as the re-sampling process by definition is probably going to reduce detail. Resampling is an averaging process which will tend to bring pixel values closer together, whereas the sharpening is somewhat the opposite, in that it finds pixels which are different and makes them more different.
Remember to do your USM last thing. If you are going to save it to web size of 640 pix. on the long leg, don't sharpen up the TIFF or JPG you are reducing from untill after you have made the smaller copy, otherwise you must learn to live with artifacts:(
