Follow the car with the camera ("panning") and the car will be sharper while the background blurs. To reduce the blur, use faster shutter speeds, but haveing everything sharp makes it less of an action photo, and more of a parking lot shot.
That brings up something I usually do. When I'm at a place where I'm taking a lot of pan shots for a while, car after car after car, I'll look for a break in the traffic, autofocus on the pavement at the racing line, then switch AF off while that point is in focus. Now I'm set, all I have to do is shoot the cars without moving myself.