Apply the High Pass filter
A professional photographer passed on this tip. It’s an ideal way to sharpen in a controllable way. Duplicate your image on a new layer. Using that layer choose Filter> Other>High Pass. Alter the slider and you’ll see the flat grey image gain detail. Convert this grey image layer to Overlay after applying the filter. If you look closely you’ll see the image sharpen. You can adjust the sharpening effect by altering the layer’s Opacity.