1. Open up your image and adjust colors and lighting.
There are many ways to achieve a good setting for your image, I'd recommend looking for some tutorials on the net. In this case I did not bother to adjust anything to make the changes we do more obvious.
2. Duplicate your background layer
Just drag the background layer onto the new layer button. Rename the new layer to "Overlay"
3. Mask everything but the skin
Add a new layer mask and mask everything else except plain skin like eyes, lips, jewelery etc.
When you're done, apply a gentle blur to the mask. (To do this, highlight the mask icon in the layers palette, usually the one right beside the icon where the content is thumbnailed and select a Gaussian Blur with a setting around 2 and 15 pixels depending on your image size)
4. Set the layer style to Overlay
5. Adjust opacity and saturation
It's so easy, all there's left to do is to adjust the oppacity until you are satisfied with the contrast and lighting, then lower the saturation to bring back the colors to their original look. In this example I used an opacity of 54% and desatured to -80:
So that's the quick way, lets go and enhance this a bit:
Click Image/Adjustments/Levels. You'll find a histogram of your image with three sliders. The sliders adjust the shadows/midtones/highlights of your overlay-layer. So let's consider some basics about the overlay mode: What it does is to lighten a pixel if the pixel on the overlay-layer is lighter than 50% gray and darken it if the pixel is darker than 50% gray. Conclusion: If we want to lighten/darken the image in general move the gray slider to adjust the medium value. If we want to reduce the darkening value of the shadow parts, move the black slider inwards, or use the white slider to darken some of the light parts. Of course you can also use the Curves-dialog box to set your overlay-layer, it's completely up to you! In the example below I gave her some extra hours in the sun :)
Add a glamour touch
A simple, yet very effective little thing to do. Apply the gaussian blur to the overlay-layer. This is the effect with a setting of 1.5 pixels in the blurs dialog box. The effect is not as effective as a direct blur would be, but notice the details not being lost, they are still there while a simple blur of the skin would just blow them away.
Paint a new skin!
That's right, let's create a completely new skin from scratch, it's so easy: Desaturate your overlay layer completely and change it's Blending Mode to "Luminosity", then create a new empty layer between this layer and the background layer and fill it with your preferred skin color. Change the blending mode to "Color" Now Ctrl+Click on the overlay-layers mask, highlight the new fill layer and click create mask. Photoshop will automatically fill the mask with the selection. Voila! Your new painted skin will probably look pretty odd but play around with your saturation settings! This is the same picture, but with a heavy blue cast you often experience taking photos in extremely bright sunlight.
At first sight creating a skin is something you might only need when a picture is really really messed up. But if you look around closely, you'll find so many useful situations to use it. A common example is nude art. Taking a picture of a nude person will almost always make the persons face look lighter than the rest of the body. This might be a good thing if you want the viewer to focus on the face, but what if you'd like to make this photo have some dreamy atmosphere? You lower down shadows and highlights and all becomes blurry and faded, except the face which whill shine brighter than everything else. Just create a new skin using the color of the persons body and apply it to the face only and it's done.
I hope you enjoyed this little tutorial!