www.eSCRAPPERS.com by Denis Germain

"Real life" examples. More control than Levels
If you used the "Levels" on this image to brighten the face. You lose the details in the Highlights or in the Shadows.
Start by moving the Midtones to a brighter value. If you go too high, you'll lose valid information of the "light" area.
Try a lower setting (we will fix it later)
To secure your "Highlights" area, you'll need to add another 'point' between the Midtones and the Highlights. Try to bring it close to the original position.
From the 'point' you could work on the lower part of your Curve. Remember to keep the Curve smooth.

Tricks for the Black, Gray, White Point Eyedroppers
When setting up these 'points' you should always remember their names. I always start with the White Point first, and I pick from an area that is white, but not pure white - more like a very light gray. Then I pick the Black Point, Once again, I avoid pitch black. And last but not least, I pick something that should ±50% gray; you should always try to pick from a neutral area (similar RGB values).
If your three picks were done right, the colors can be very nice, but sometimes require an extra boost from the Hue/Saturation or Contrast/Brightness Tools (Be careful not to overdo it)
When using the eyedroppers, you can "see" the results of your pick with the Channel drop-down menu. Use the "RGB" to modify the brightness and or contrast.
Other possibilities: 1) You could also place a grayscale chart in your camera bag, to do a manual white balance before the shoot. 2) take a photo of it so you have an on-location reference photo. If several photos were taken in the same conditions simply save your curve and use it on all the photos from that session.

In many cases playing with the auto color options will generate great results. CTRL+M to open the Curves dialog box, then choose Options. Choose Snap Neutral Midtones and pick one of the three Algorithms.

You can also get amazing results with Power Retouche Photoshop Plugins