When I was younger, so much younger than today, I never needed anything else than unsharp mask amount 120 and radius 1 pixels and no threshold in any way. But now these days are gone, I'm not so self assured, Now I find I've changed my mind and opened up the filter menu (and start with amount 75 en try and look at the 100% preview) And now my life has changed in oh so many ways, My old habits of using just one set of values seems to vanish in the haze. But every now and then I feel so insecure, I know that I just need to look at the pictures, try what works, zoom out, undo, retry like I've never done before. Sometimes it work to use the filter twice with small amounts. Sometimes on large pictures you need to increase the radius. And when you just ruined the muscles in your hands by clone stamping every speckle and dust particle, unsharpen mask can reveal a whole new world of fresh dust. That's when the threshold comes in handy. Or when there is a hefty grain in the photograph. Rule of thumb 01: if you can see the lines around contrasting areas when you zoom to 25% (presuming you are working on material for print, else you can stick at 100%) your unsharp maks is too much. It needs to work almost subconsious: sharpening the look of the image, without revealing how it's done. And, remember, it is not a miracle filter, it won't sharpen images that are really unsharp. Rule of thumb 02: if people don't know the song 'Help' by The Beatles, they will think I am raving mad when reading the first part of this answer. And even if they actually do know that song, they may still have their questions.