Hi all, I wanted to share a very useful Photoshop action I came across from MCP Actions. It's called Touch of Light/Touch of Darkness. It's free to download, too.
I'm no Photoshop expert so I'll describe it in layman terms. Basically, upon running the action, it creates two new layers; one for light and one for dark. Then you just select the layer you want and paint over the area you want to apply the light/darkness to. Use a soft brush with 30% opacity (their recommendation).
As it's done on layers, they can be turned off if the effect doesn't work out well. I find it's really good to bring some light into shadowy areas or to shadow over too bright areas. On my Apache photo (below), among others, I painted shadows over the three people in the foreground to minimise their presence.