Rim lighting is an effective way to visually separate the subject from the background especially when it might be hard to distinguish one from the other because of an intersection of similar tones. The rim (or kicker) light creates a thin highlight around the edges of the subject, partially outlining her with bright light. A skillful application of rim lighting can give the image a crisp and polished look. I go into depth about rim/kicker, and side lighting in my eBook, Lighting Guide for Portrait Photography, if you'd like to learn more. But here, I want to talk about how to use a couple of flash units to create an easy rim light and avoid a potential problem with flare and light spill.
To create rim lighting, you'll usually need to point a light source at an angle that strikes the subject from behind and to one side. When the light source is narrow, or otherwise modified or positioned so that a large amount of it does not also reach the lens directly from the source, you're in good shape. Otherwise, you might end up with the following effect.
In the image above, I'm using two small flash units without modifiers, to create rim lighting on both sides of the subject. You can see how the light does indeed create a strong rim around the edges of the subject, but it's also bleeding into the lens, resulting in a hazy light effect. Note: I haven't applied the main light yet in this example.
To rectify this, we need to block off the light reaching the lens directly, while maintaining the way it's striking the subject. This can be done with modifiers like grids or shoots, but simply flagging the light off (blocking the line of sight from flash to lens) will do the trick. This next shot shows the result of flagging the rim lights and applying the main light.
In the image below, you can see I've placed two V-flats in such a way as to allow the light from the flash units to strike the subject while shielding the lens from the direct light. This side view shows one of the flash units, but the flashes are not visible from the actual working camera position.
Personally, I don't use a lot of rim lighting unless it's subtle, or it's the type of shot that calls for it, like something decidedly commercial. But I will take advantage of a similar effect that helps to create that separation; reflected light that produces a subtle highlight on the shadow side of the subject.
In this close crop (above) using a medium-sized softbox in a basic one-light portrait setup, the background is dark and the subject's shadow side might have blended and disappeared into the background had it not been for two things:
1) The softbox is angled in such a way as to spill a little light onto the background so it does not go completely dark just behind the subject from the main camera angle.
2) The light from the softbox is also reflecting off the white surfaces of the room, including the floor, creating a subtle highlight on the shadow side of the subject.
Given this last example, you can see that a rim lighting effect doesn't have to be created with a direct light source, or even a flash/strobe; any reflected light will do as long as it can be recorded during exposure. Keep in mind that it's not absolutely necessary to apply rim lighting, even when part of your subject is disappearing into the background. After all, that might be the look you're going for. But if the image calls for it, rim lighting is an easy technique to employ.