Another trick using the selections generated from Channels as adjustment layer masks.
- Load the RGB channel as a selection by cmd+clicking on it in the Channel palette.
- Create a new Curves layer, automatically creating a layer mask with the selection.
- Alt+click on the selection and invert it by hitting cmd+i.
- Apply a curve to the mask by hitting cmd+m.
- Move the leftmost point (the black point) to about a quarter of the way in. Move the rightmost point (the white point) to about three-quarters of the way in.
- Right now, the mask is adjusted to just the midtones of the image. Return to the normal view by hitting the eye icon on the curves layer.
- Pull up on the curve at about halfway to apply a bit of fill light to your image’s shadowy areas.