Elevators have numerous safety features. There have been very few recorded incidents involving death from plummeting elevators. In general, it is highly unlikely for a cable (also called traction) elevator to fall all the way to the bottom of the shaft. Moreover, the compressed air column in the elevator hoistway and the car buffers at the bottom of the hoistway may keep the forces of the impact survivable.
1 Flatten your body against the car floor.
This should distribute the force of impact, rather than concentrate it on one area of your body. (Standing may be difficult anyway.)
2 Cover your face and head to protect them from ceiling parts that may break loose.