The first thing I do with any image in Photoshop is always to adjust the levels (as per the link) and then the curves. (That's possibly more important with a colour image than a mono one but I'd still advise it).
With my version of Photoshop (6.0), I get to the relevant option via Image > Adjust > Curves, but pressing Ctrl and M together also works. That brings up this:
http://pe-images.s3.amazonaws.com/photo-editing/levels-curves/photoshop-cs2-curves.gif
Most images are improved by 'grabbing' the line (= click & drag) to change it to a gentle 'S' shape, possibly like this:
http://www.kenstone6.net/fcp_homepage/images_photoshop_elements_9_stone/photoshop_elements_9_stone_72.jpg
(although what's shown there might be an excessive level of adjustment for many images).
In some cases though pulling the middle of the curve up and to the left (to create a wide 'C' shape) is what's needed, whereas sometimes the reverse (pulling the middle of the curve down and to the right) produces the best result.
I've been doing it for so many years that I can simply look at most images and intuitively know what's needed for any particular one but (as I did at first) you'll just have to experiment a bit!