I've never tried it either, but i think the best idea would be to focus on the point where you actually want to take the photo, like the apex of a bend, and then pan with the car as it comes toward the place, then snap when it's there, that way you should be in focus, with lots of nice background blur. In theory anyway...
It could be a good idea just to try snapping cars on a fairly fast road near you to try out/practise techniques.
Cheers,
-Ads
"We used to come down, doing about 180mph. We take off over the bridge and change down to 4th gear in the air. Go through the righthander with the car drifting, and let it swing out through the lefthander and then slide out to the wall, where we simply stopped it with a flick of the wrists"
-Vic Elford, on Maison Blanche in a Porsche 917
UCP's Most Hardcore Armchair BTCC Fan & Anthony Reid Supporter