Hmmm, try Filter- Enchance -> Sharpen, do not use a setting higher then 50, or you'll create noise, but this is the best way: Add an "Unsharp mask", set it to your likings, do all the stuff you want to your image and sharpen after you're done with your image, including resizing, always sharpen after you're finished!
Go and see the Photoshop tutorials thread, I've got 5 GIMP tutorials in my gallery, approved by other members 👍