One way i do it is to make a selection of the area (using the lasso tool) then call up the IMAGE --> ADJUST --> BRIGHTNESS/CONTRAST dialog box and move the sliders around to were i want. This will affect only your selection.

You can also make your selection and select a brush, change the blend mode of the brush to "colour burn", drop the opacity down (20% or so) and paint away on your selection and this will affect (darken) only your selection. By dropping down the opacity, you can "build up" on your sharpening/contrast with more control. Each pass over will make it darker.

hope it helps...
:)
retrocity