Step One: Image to Touch-Up
Obviously we need to start with something. I'll be using the photo below, which I downloaded from sxc.hu's stock photographer remind.
Step Two: Set Color & Start Painting
More often than anything else, this technique will be used when you have a photograph of someone inside a house - there isn't a lot "wrong" with the photo I'm working with but we're going to play anyways.
The first step here is to set your foreground color. Think of natural lighting colors - a very light, almost-white yellow, oranges, etc. Make sure that any color you use is nearly white, but not quite.
Then, grab a brush - it doesn't really matter what one, as long as it's not too large. Create a new layer (Layer, New, New Layer) and start painting the areas that could use highlighting.
Step Three: Layer Mode
After you've painted a few key areas, it's time to see how it looks. Go to your layer mode drop-down (it's right at the top of your layers palette, and currently says "Normal") and choose "Overlay".
You may want to lower the opacity and/or clean up the painting with an eraser.
To add more highlights on other areas, repeat the process using a slightly different color on a new layer. This will start building up a beautiful series of highlight and shadow.
