Open up your picture and duplicate the layer.

Choose the lasso tool and set the feather on 15 px or more if your image is bigger.
Now I'm going to start off by circling around the eyes and and selecting image/ajustments/levels.

These are the settings I have chosen for the eyes you can choose the best settings for your image.

Now I am going to do the teeth,hair and top with the same technique.
This is what I ended up with it only took a few minutes to enhance this image.

This is it after. Rollover the image to see what it was like before.