Large rubber chocks both ahead an behind both wheels. I cheat and have ropes attached to the chock to make it easier to retrieve them. They get tossed into the back of the truck so that thy are easy to get if we park on a steep incline. My wife throws one on each rear wheel of the truck, I let the truck roll onto the chocks and then put it in park.
I saw one gentleman parked on a steep road and his tranny was really stuck in park and could not be released. I pulled out my tow strap and released the pressure on the tranny and he easily shifted out of park.