In backlit scenes (background is much brighter), the subject will often turn out too dark because the camera is measuring the brightness of the entire frame. Spot metering is a good solution to this problem.
Without getting into too much techno speak, spot metering measures only the small area in the center of the scene allowing you to measure only the light bouncing off of the subject’s face and expose properly for that. (Spot metering works best on people with light or medium skin tones.)
If you’re shooting an open air festival with the band in front of a bright sky, change the camera’s metering mode to Spot Metering. Put the subject’s skin in the center of the frame and press the Exposure Lock button. Focus, recompose and shoot. Check your LCD to make sure you’ve done this correctly. Lather, rinse, repeat as necessary. If your camera does not have the spot metering option, try using center-weighted metering.