We were at Spa last year and we took a Canon EOS as well as a sony digital camera - never having taken motorsport photos before the digital camera was great practice as I could delete all the photos of empty track, chain fences and tail ends of cars!!
The best tip I found was mentioned by KC above (point 6) keep following the car until you see it again in the viewfinder!! Shall probably be able to take a few better ones at Le Mans this year I hope....