I follow the car with Ai Servo and I trigger when in I believe it's the right time. Like this you can wait for the perfect moment and shoot.
With slow camera's you need to focus on a corner and wait untill the car passes your focuspoint.
I also play with focus points on my 40D like if I have the leading car on the right side and the one following on the left, I quickly put the right focusing point on so the first car is in focus and not the second (or the ground).
I also experiment with light measuring methods and sometimes (mostly jumps) I manually set the aperture and shutter speed.
I also prefer to photograph good racers that hold their line instead of people driving all over the place
another tip: open your eye that is not in the viewfinder to locate the car that is about to pass the area you are shooting (a couple of corners mosty) or to locate a battle between a couple of cars that is important to have on camera.