Script-Fu -> Sharpness -> Sharper -> High-Pass Sharpen
This is a an adjustable script,
that sharpens the high-end/highlights in an image.
Script-FU > Sharpness > Sharpen > High-Pass Sharpen
Dialog
Original Image
Result:
Tweaking results...
...and usually I tweak the results by opening the layers dialog and raising the opacity of the sharpened layer.