For more reading see:
Panning is a great technique for action and, once perfected, the main subject will be sharp against a blurred background. The idea is to follow the subject as it passes in front of you and continue to follow it as you press the shutter and even after the shot is taken.
If you pan at the same speed as the subject it will appear sharp against a streaking blurred background. To ensure smooth results keep your feet still and rotate the top half of your body as you track your subject. Prefocus your camera at a point where your subject will pass to ensure the picture is sharp. Also ensure the background isn't too light and doesn't have shapes as this can create ghostlike effects or streaks in the image. A darker background is better
Samples on Flickr