First, start with a well lit, clear picture. Scan it at about 300 dpi. Using PS, I would do Auto contrast, auto levels. Then, go to filter, and click on dust and scratches (I think that's the name of the filter). If you're using a 300 dpi scan of a 4x6 photo, set the # to 1 or 2 pixels. Then tweek the image for color and contrast manually. If you need to darken the outdoors, use a screen. Don't try to change individual colors.
From there, resample to about 72-96 dpi. Then size it to what you need. Save it, and I would suggest nothing lower than 8 for quality (if saving as jpg).
There is more of course, but this should be a basic start to getting better quality digitals from printed originals.
Good luck,
Marc
Webpager2