Always sharpen at 100% view.

Except for Sharpening in Scan or “First pass” digital captures, Sharpening should always be the last step before printing. (Always size for printing before sharpening)

Optional First Pass on Digital Capture(no sharpening in camera) or after scanning; Amount 300%, Radius 0.3, Threshold 1 or 2. then “Fade Luminosity” (see below)
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Amount, Radius, & Threshold Explained: Sharpening means increasing contrast along the edges of the subjects in a photo.
Amount: degree to which edge contrast is increased (higher number, more contrast).
Radius: width of Halo. Higher number means larger Halo. A little goes a long way.
Threshold: Noise reduction control, limits sharpening which helps avoid halos and noise. (Helps over sharpening of subtle textures, ie; portraits).

Sharpening for Print or Slides
High Detail Images; Start with Amount 300%, Radius 0.7, Threshold 0.

Low Detailed Images (Portrait/soft Flowers); Amount 300%, Radius 0.8, Threshold 8.

Note: If more sharpening needed, increase Radius; if less, reduce Amount. Finally to mitigate noise, adjust Threshold
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Sharpening a Selection; (to leave the sky out or to avoid grain/noise in an even colored area eg.), Select sky (with magic wand for example) Invert selection, then Feather the selection by a few (2 pr3) pixels before applying unsharp mask.

Fade Luminosity; To avoid potential color changes caused by the unsharp mask filter you can “fade luminosity’. Immediately after sharpening, select Edit>Fade from the menu bar. Change the bending mode from normal to luminosity and click OK.
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Optional Method for sharpening with unsharp mask
Amount 300%; Radius 0.2; Threshold - Natural History images use 0, for images with big sky use 2-3, and for portraits use 4-8. With this option you may sharpen more than once, decreasing the Amount each time.