The dodge and burn tools (shortcut O) perform a similar function to their darkroom namesake and are ideal for localised tonal control. Also under this menu you will find the sponge tool which can be used to alter the saturation of local areas in your image.
Photoshop 6 displays the options for the dodge and burn tools under the menu bar when the tool is selected - if you are using Photoshop 5 double click on the the icon in the tool box to call up the options palette. Photoshop sets the exposure default for these tool at 50%. You will need to reduce this figure to around 10% or less because much like the darkroom technique you need to gradually build up these effects.
You can also choose which tonal areas you want to work on by selecting shadows midtones and highlights from the drop down menu together with the brush size.
The burn tool lets you darken an area of an image, and works in the same ways as the dodge tool as shown above.
The sponge tool can either be used to desaturate, which remove saturation and contrast from an image (see above), or you can choose to boost the saturation saturate and add contrasts (see below). These options can be found in the mode drop down menu. Start with the pressure set at around 10% and slowly build up the effect.
With all these tools a subtle approach will give the best results and as with many of the Photoshop tools you may find you get better results with the image greatly enlarged.