As above, you've got the basics in the shot. A fraction of a slower shutter speed would give you give that little more blur.
One thing that strikes me with the image though is its overall framing. Even though there is blur to the shot, the overall shot doesn't really give a sense that the bike is going anywhere. Is it coming into the shot, going out of frame, through the frame, coming at you?, going away from you? … none really, its kinda just there … and all that's there is the bike …. which leads on to:
A quick google search of "car panning shots" brings up loads (some good some bad) … but for a lot of the good ones you'll generally see that the subject takes up much less of the frame and there is more background to the image. By not zooming in so much on the subject, you'd get that effect. This would also have the benefit of being able to do a crop later on which places the subject where you want it in the frame. I.e. give it an angled look, place the subject to the left or right of frame etc etc.