Hurling yourself from a moving car should be a last resort, for example if your gbrakes are defective and your car is about to head off a cliff or into a train.

1.  Apply the emergency brake.  This may not stop the car, but it might slow it down enough to make jumping safer.
2. Open the car door.
3. Make sure you jump at an angle that will take you out of the path of the car.  Since your body will be moving at the same velocity as the car, you’re going to continue to move in the direction the car is moving.  if the car is going straight, try to jump at an angle that will take you away from it.
4. Tuck in your head and your arms and legs.
5. Aim for a soft landing site: grass, brush, wood chips, anything but pavement – or a tree.  Stunt people wear pads and land in sandpits.  You won’t have this luxury, but anything that gives a bit when the body hits it will minimize injury.
6. Roll when you hit the ground. 

Source: The Worst-Case Scenario Survival Handbook by Joshua Piven and David Borgenicht