Normally, when we’re lighting a background, we want to light the background and the subject separately, which is one reason why we try to keep our subject 8 to 10 feet from the background (well, that, and we don’t want to see the subject’s shadow falling on the background behind them, and creating that separation lets their shadow fall on the floor, instead of on the background). However, if you only have one flash, and you want your background lit, just move your flash close enough to the background, so that the light from your flash can reach it. The spill from that one flash will light the background, as well as your subject, if it’s close enough. This works best with large softboxes, like Westcott’s Recessed Mega JS Apollo 50×50″ softbox or their 7′ shoot-through parabolic umbrella, simply because they can cover more background area with their size, but it can still work with smaller 32″ softboxes (like the Westcott Rapid Box Duo) or even 26″ softboxes, like the regular Rapid Box, too. So, next time you’re stuck for a background light, move your front light (and probably your subject along with it), closer to that background and let your flash do “double duty.”

From Scott Kelby’s The Flash Book.