Buy a heavier shotgun. Shooters try moving down to a 20ga thinking recoil will be less. A 20ga shoots lighter loads (generally) than a 12ga, but a 20ga usually weighs less than a 12ga, so you still can have pretty significant recoil. No magical solution, you either have to shoot lighter loads, or get a heavier gun.
I’ve got a pound of lead in the hollow of my Winchester’s butt stock