Step 1: Grab and Image and Make a New Layer
Make any adjustments to this layer... adjust curves, light levels etc. This is a good time to clean up skin blemishes etc. (I did not do so for this picture, this was just a quick write up as an example).
Step 2: Create a New Layer Based of the Adjusted On
Now, using the eraser tool, remove bits of the top layer (the B&W; one) to reveal the color(s) below. Make small strokes and go slowly, you can always use the undo feature of your photo editor to fix mistakes.
Step 3: End Result
I always do these things at the end of a session so I can utilize the history brush should I need to.
Other things to highlight:
Lips, eyes, hair, jewlery, props, etc.
But be warned, too much is a VERY BAD thing. And sometimes it just looks evil or puke awful. Simple is best and use any bit of artistic judgement or even get a second opinion :P