Alternate and simple way to enhance dark photos.
Here is a great way to lighten dark parts of image without blowing out highlights
http://spectacled-bear.hubpages.com/hub/How-to-brighten-up-dark-photos-without-blowing-highlights-using-FREE-software
This photo was too dark in the face and simple brightening would blow all the light areas.
After lightening with layer masks I preserved the lighter detail quite well. I could have completely not lightened some areas by painting out the mask with black and I probably was too extreme in duplicating for lightening but you get the idea.
Masking for hair or detail: Photoshop using channels