A couple of years ago I started scanning old family photos. This Polaroid was taken sometime in 1976, just after Sean was born in Leaf Rapids, Manitoba. If you click to embiggen, you'll see that the scan is marred by several scratches and countless dust motes. This image has been shrunk so that I don't use up all of my free Blogger space any sooner than I must; at the original scan size, it looks even worse.
Here's the same image after about an hour and a half's worth of correction. First, I corrected the levels as Jeff Shyluk taught me last year, trying to approximate a nice bell curve on the histogram. Then, I used the clone stamp and the spot healing brush to erase the dust motes and scratches. Finally, I used the burn tool to darken the image; this may have been a mistake or at least unorthodox, I don't really know. But I think it's an improvement. I probably should have tried to use the unsharp mask filter too, but my confidence with that tool is still minimal.
One thing's for sure: to minimize headaches later on, you should always keep your scanner glass as clean as possible. Every dust mote, fingerprint and imperfection on the glass will show up on your scanned image. I've learned this the hard way.