Meet the GIMP #6: Selective sharpening
Sharpening should always be the very last thing you do with your image.
Chris Marquard explained sharpening in Photoshop Corner No. 4 in his Tips from the Top Floor podcast.
I got the described method from the Gimp tutorial page.
Wikipedia describes the underlying process in this article about the unsharp mask. The links at the bottom of the page are very informative.