Well, as far as what settings to use, the simplest answer is "it depends on the picture". Most people tend to find that most of their images...even the really sharp ones, can stand to use at least "a little" sharpening, and like virtually everything else in Photoshop, there's no one singel "best" way to do it.
To give you a break down on Unsharp Mask (and someone feel free to correct me if I'm wrong here as I don't usually use USM very often myself), the controls are pretty much as thus...
(I'm going to start from the bottom)
Threshold - This determines the difference between the pixals needed for the effect to be applied. I tend to find that a larger threshold produces a more subtle result across the entire image where as a smaller threshold tends to produce a more pronounced effect in the high contrast areas of the image.
Radius - The determines the number of pixels surrounding your target area that are actually effected. A smaller number here will generally produce finer results in detail areas, where as a larger number tends to effect a much broader range around your high contrast areas.
Amount - As the name implies, this determines the amount of effect thats actually applied.
Personally, I tend to start with the threshold around 8 or 10(ish) and I use a small radius...usually less then 2 pixels, with the amount around 100% to start with again depending on the image and then fine tune from there. Something to keep in mind is that (again generally speaking) it's usually better to do any extreme sharpening in multiple passes...instead of trying to do 200% for example, apply 100% twice (with lower radius and threshold settings of course) and remember that sharpening is like cooking...you can always add more spice to something later, but it's really hard to take it back out if you add to much.
Now a couple of things to keep in mind...first and foremost, if the image is really out of focus, Unsharp Mask (or any other PS tool for that matter) won't "fix it". USM creates the illusion of sharpness by increasing the amount of contrast along edges and lines, but if the image is simply blurry, there's no magic fix to recover it.
Secondly, I would highly suggest that any sharpening you do, you do to a seperate layer and not to the "background layer". This way if you do over-sharpen the image, you can go back and change it later (and keep saying to yourself, "Layers are my friend, layers are my friend, layers are my friend" LOL!!!).
Third, again there's no one single right or wrong way to actually sharpen an image. Some folks I know go all out and apply sharpening selectivly to the individual channels and then there are folks like me that use different methods for sharpening altogether...I use the High Pass filter and then change the blending mode for the sharpened layer to "over lay"...I tend to find that with animal/critter pics, it tends to produce less halos from the sharpening process then Smart Sharpen or USM. You can do similar with the emboss filter as well. This is simply a matter of experimentation and trial & error to see what works best for you.
Again there is no magic formula...every image is different and is going to require different amounts of sharpening in most cases and how much you actually need to sharpen a given image will probably require at least a small degree of experimentation.
I hope this helps!
Bright Blessings & Gentle Breezes,