There are several ways to get separation. First is by using rim lighting, also known as backlighting or edge lighting. This creates a glow around the edge of the subject and causes him or her to stand out. The image shown here is from the Lowel website. For a demonstration of where to place backlights, visit the site. Another way to get separation is to make sure the background is not too bright. Using "egg crates" on your softboxes or barn doors on your open-faced lights, you can keep the light from spilling onto the background. A few highlights on key background elements helps bring the background to life, but too much light is a distraction from your subject in the foreground. Of course, this is not a hard and fast rule. A darker subject may require a lighter background. (Note: The "Chiaroscuro" principle is a good one to know. It says that the brightness of the background should be in direct opposition to the brightness of the subject. For example, if the key light is on the left side of a person's face, the background would be darker on that side and move to lighter on the side where the fill light is placed.)
The most obvious way to create separation is to actually leave a little space between the subject and the backdrop. Instead of shooting someone on a couch against a wall, try seating them in the center of the room. While this does not necessarily create separation by itself, expecially if you are using a longer lens, it does allow the background to go out of focus with very little effort. A sharp subject stands out very well against an out-of-focus background.
