I take this layered approach:
Say I want to add some light, I make a new levels adjustment layer, adjust the white point a little, but mainly increase "brightness" by moving the gamma control. I make the areas I want to be brighter look "correct" not, worrying too much about the rest of the image.
I now set the adjustment layer to black so it has no effect, then begin painting in white, gently, in the areas I want brightened.
I can now tweak the overall amount of light I'm adding by adjusting the layer opactity, or tweaking the levels operation.
I then apply a similar layer, tweaking black point and gamma for darkening.
Advantages with this method is that you can do gross and subtle effects quite easily. I find there's limits to how much light you can add with the "overlay" method, and often found I had to stack multiple layers to get the effect I needed.
Graeme