Panning with moving objects

[Alien]

Okay I'll start us off in this forum . . . a while back I was at a motorcycle race at the local track (if you call 3 hours drive local), and trying to get some pix of the bikes that showed the speed, so I was doing some panning. Now when I got the slides back, I found that maybe 5% of my pix were usable. The others were either blurry top to bottom, showing that the camera had moved vertically (although I had it on a tripod of course), had the bike in a poor position in the frame, or weren't tracked accurately so that the bike was blurred front to back. This was with a 300mm f/4 lens.

So anyone have better success than I, or any tips for good panning shots?