Using a properly fitted, purpose built child carseat which meets current safety standards (marked with an 'E' and an '03') and is appropriate to a child's weight and height.
Thousands of kids are killed or injured each year in crashes resulting from improper car-seat use or installation, with the National Highway Traffic Safety Administration further reporting 33 fatalities last year from heatstroke as a result of children being inadvertently left in cars (we can only assume the latter occurred while their parents were too busy talking on their smartphones to notice).