Using sharpening tools can significantly improve your images -- but you need to know what you're doing. Photoshop expert Helen Bradley has a useful guide to the basics of sharpening technique.
Sharpening isn't a one-click fix, but with a little patience you can get impressive results. While the post talks through some common Photoshop techniques, the principles apply to any image-editing suite. Check out our Hive Five of image editing tools for more options.
Understanding the basics of Sharpening in Photoshop [Projectwoman]