There is many more people here better at both taking motorsport photos and giving advice than little old me, but if it help I'll share my thinking on getting a decent shot.
Number one rule: don't kill yourself, putting yourself in a dodgy spot and taking huge risks is not worth it for a shot. On a lot of my images it looks like I am in a very dangerous area, but in fact I'm behind a thick wall, trees, or I have somewhere to run to, *ALWAYS MAKE SURE YOU HAVE A ROUTE TO RUN*
This looks dodgy, but I'm miles away shooting at about 500mm, and the shot is cropped, I'm also next to an open gate, giving me space to run:
Before an event I always get my hands on the stage maps and drive the stages myself if they are on open roads, this way I have a better Idea on what a car will do at a particular bit, ie: if I swerve, dip, jump at normal road speeds, I know a rally car going at warp speed will also do something dramatic. on closed roads, or forrest, I'll walk them, you still a good idea.
With regards to the actual shot, I try and fill the frame with the car, using backgrounds only if it add's something to the shot, for example, fans all looking the same way:
You'll also notice on that image is that there is a little space for the car to move into, rather than a load of empty space behind it.
Blur is definatly a friend, combined with a flash, you can get some cool effects, I would not recommend practicing flash on passing traffic, maybe getting someone to roll a ball across your lounge floor while you shoot would help learn about all the settings.
I'm still learning how to pan, getting that car (or anything else) nice and sharp while getting everythign else blurred is all about practice, practice, and then maybe a little more practice, if it helps, I take a strong stance, tuck my elbows in, and swivel my hips, not my arms. try your setting out on the "Zero" cars (safety cars going at race speeds at the start of a stage)
The only occasion where a freezing the action is a good idea is either on water splashes where getting millions of water drips flying off everywhere can be an interesting shot (blurring them too is a nice effect, as shown on the blue scooby) and jump shots, the problem with slow shutter speeds on these is that the car is not only getting side to side motion, but also up and down, high shutter speeds is the priorty, boost your ISO, largest aperture, anything that helps you freeze the action ... try it on a bouncing ball.
A lot of people ask how I focus when the car is going so fast, I know some of the top guys "Follow focus" = manual focus while following the car in the viewfinder, and its well bloody hard to do, I prefere pre-focus on a spot that I know they will cross, a pebble, or the edge of a puddle ... it doesn't always work, but you cant expect every single shot to come out. I'm experimenting with the camera built in follow focus, trying to keep the crosshairs on the target while panning, its good, but still getting the hang of it.
Follow the car through the viewfinder everywhere, if something goes pear-shaped you'll already be taking the shot.
Hope some of this has been of use.
Regards
Kris