Put the dash centre vents in their mid position, so the air blows straight out, then direct the airflow up as far as it goes towards the roof. Then when the cool air hits the roof it gets deflected back down towards the rear seats.
I have all my vents pointing upwards as that is where the hot air is. The cool air then naturally falls cooling the whole cabin. I have had no problem staying comfortable in the hot weather and never had a complaint from rear seat passengers. I tend to leave the Climate Control at 20 and Auto all year.