These are all good tips.
One thing I look for is never to have a pole in the background. Telephone pole. Power pole, etc. This is a key rule for photographing people, but a good tip for cars as well.
Keep the background simple.
Tires straight.
Move up to the car, or better yet if you have a zoom move the focus in to the car.
In some settings shoot with high speed and low apperature...sometimes you can soften the background by having it out of focus while the car stays in focus.
Before you take the shot look for excessive reflections on the body or glass..