This is something I've known how to do since....well since like Jr. High, but I rarely ever actually do it. I sometimes get lazy with my editing and don't tend to go past a bit of cloning and of course the obvious color/contrast tinkering. Lately, I've been following some photographers who go past that basic editing and start layering. This has convicted me to break out of my lazy streak and mess with layering more often.
Layering isn't hard, but it can be somewhat time consuming. The advantage of this technique, though, is being able to adjust your color/contrast differently with each layer. Below is a shot I broke into 2 simple layers. One is for the subjects and the other is for everything else. Backgrounds (especially skies) tend to look much better with higher contrast, so it's a great technique to whip out when you've taken some portraits outside. Don't go overboard though or it might look too fake!