Car is a fun sign that babies love. Place your hands around an imaginary steering wheel. Then turn the wheel from side-to-side as if you were driving. (At least that is how you would drive if you constantly swerved from side to side like a mad man).
Use car whenever you are about to embark on a car trip. "We are about to go in the car." Or when you are taking walks, point out the cars that pass you by. Many infants have toy cars that you can use as an opportunity to learn the sign.
Cars are present in the day-to-day experiences of every baby. Teach the sign early in association with trips to the store or family travel.