One trick I found over the years is how to make a slightly fuzzy image LOOK like it's sharper. This doesn't always work as well as I hope, and for images that are downright blurry, there's really no help. But some soft images can be faux-sharpened. I used to do this with oils and colored pencils on real prints I was working on. When I went digital, I just kind of addapted the idea.
1. In soft images, there is what I call a "blur zone" around critical areas. If the edges of that blur zone are made harder, the image appears more in focus. The critical areas are eyes, mouth, ears, fingers, fingernails, etc. In other words, features...
2. Using a soft paintbrush set at about 10% opacity, start eliminating that "halo" around the eyeballs. You don't want to make the edges look freakish. You just want to eliminate that haloing or shadowing that is what causes the fuzzy look. Adding a slight ring of dark at the iris' edge is also sometimes helpful.
3. Lightly cloning also works in some areas, such as around ears, hair etc. I often clone to define the area between subject and background.
4. The last part of the trick is to enhance the highlights in the eyes. Most soft pictures either don't have these highlights or they are very dull and lifeless. With your brush set at a low opacity, lightly dot in a white highlight. Not too strong and not too big, but with at least one pixel of pure white surrounded by off white ones.
Just doing the eyes alone can add an amazing amout of perceived "sharpness" to your image. :)
The trick here is to remember that more is not always better. If you overdo this technique it ends up looking freakish instead of sharper. But done lightly and properly, it can really enhance an image.