This was done using Photoshop CS2, but it should be easily translatable. :)
First, start with your image. Mine is of Alyssa Milano.
Do your basic adjustments with Auto Color and Auto Levels, or whatever you choose. I decided (as I usually do) that I wanted to adjust the Brightness/contrast, and did so. My settings were:
Now, I'm a huge fan of rich, highly saturated colors. So I increased the saturation of this image.
But eugh! Look at what it did to Alyssa!
She looks like she's trying to audition for a part in the next rendition of Charlie and the Chocolate factory. This will not do!
Create a new transparent layer and set it to Soft Light.
Next, take a basic skin tone. I chose FFEAE3.
Next, set your brush to 40% hardness and 40% opacity. Brush over her skin (and don't worry about being perfectly neat here. It doesn't matter that much). Keeping the opacity low makes it so that the changes are subtle and not too much at once. You can always build up the color as much as you want.
Questions, comments, at Inama Nushif