An area that has too much light will make it difficult to slow your shutter speed down enough for a panning shot. (Photofocus)

Make sure you press the shutter when the subject reaches a mid point along your panning track to ensure it's in the best position and try to follow without moving up or down to prevent subject blur. (ePHOTOzine)

Panning is a means of creating a visual sense of motion in a photograph. (The Lawrence Journal-World)