Many times when an accident occurs you are not exactly thinking as rationally as you would like. If the other driver was at fault or you were a passenger in a car involved in an accident, then make sure the police are called to investigate the accident scene. The independent investigation of an officer at the scene provides a lot of evidentiary weight.
If you can, get a witness or someone to take photographs of the collision. This is helpful to the lawyer to demonstrate the nature of accident and magnitude of the collision, especially if your case has to go to trial.