I mostly use a masked curves layer as described by rmklass. I sometimes use more than one to to affect the whole image or different parts of it in slightly different ways. I will fairly frequently create a luminority mask to use as a starting point for the mask. I can then lighten or darken certain areas or the mask, and apply levels or blur to the mask itself, to get the right degree of effect at the right places.
For simpler or more general lightening or darkening, I like to use a blank layer set to soft light blend mode, and filled with 50% gray. Painting with a lighter or darker brush on this layer has the corresponding effect, but does a nice job preserving contrast, and is more subtle than using multiply or screen layers, in my experience.
Last edited by dsjtecserv; 09-01-2009 at 12:35 PM.