Sharpening an image depends entirely on the image. Some take it well, others don't. Again, trial and error is the best teacher here. Try these experiments to see if anything makes sense for you: duplicate the image layer, then apply a filter (try 'gaussian blur', others you might try are 'median', 'high pass', and any others that strike your fancy, not all at once, but on different experiments). Use the filter far too strongly, so that it totally ruins the image, then select the 'multiply' blend mode and start turning down the opacity. Other blend modes work better with different images, so try them all (I use 'soft light' and 'hard light' a lot).
You can also simply apply the 'unsharp mask' filter to the new layer and then follow the same steps. All of these can be further refined by using the selections I mentioned earlier to choose what will become this new layer.