Open your image in Photoshop and create a new layer that is a copy of everything you’ve done so far. You can do this by hitting CTRL-ALT-SHIFT-n and then CTRL-ALT-SHIFT-e on a PC, or COMMAND-OPTION-SHIFT-n and then COMMAND-OPTION-SHIFT-e on a Mac.

Choose the “high pass” filter from the Filters - Other menu. Apply a high pass filter to your new layer. Choose settings that reveal the edges in your image but not too much. Values will vary between 1 and 10 depending on the size of your image. Try to avoid choosing values that reveal colours. If you really need the high pass setting that high then consider desaturating the layer after you’ve applied the high pass filter.

You should now have a grey looking high pass layer with the edges of your picture visible. Add a layer mask to this layer by going into the Layer menu and choosing Layer Mask - Hide All. This adds a layer mask to your high pass layer that is filled with black. Your grey high pass layer will seem to vanish.

Select the brush tool and choose white as your foreground colour in the colour chooser. Now paint white over the areas of your image where you want the sharpening applied. You’re painting white onto the layer mask. This reveals your grey high pass layer in the areas wherever you paint white.

Once you have your grey high pass layer showing on all the areas of your picture that you want sharpened choose “soft light” from the blending mode drop-down at the top of the Layers palette. This sets your high pass layer to soft light blending mode which has the effect of sharpening those areas where you painted white in the layer mask.

For a stronger sharpening effect try out “hard light” or “vivid light” blending modes or copy your high pass sharpening layer.

For more information on this topic and on sharpening in general, you can also listen to TFTTF show 88 - PSC04: sharpening.
Or you can take a look at this topic in the photoshopcorner.