Instead of using your flash to provide the primary source of light, shoot for a good ambient exposure and accent your subject with a low power off-camera flash. Shooting at a relatively slow shutter speed (1/30 sec or even 1/15 sec), with a large aperture, you get a nice ambient balance that's a perfect base exposure. The lower power flash allows you to accent your subject without changing the moody feel you get from natural light. If you shoot your flash at 1/64th or even 1/128th power through a small snoot (a cone of paper will do) that focuses the light on your subjects faces, you can retain the overall feel of the image while creating a nice moody accent on your subject.
Photo: Matt Adcock