Forehead against the seatback in front with hands behind/across the back of your head, holding it in place.

The goal is to limit the distance between your head and the hard impact of the seat in front of you. They're designed so that at 16G you shouldn't get cut too badly but that's not going to stop the bruising or concussion. Limiting the distance your head swings free before impact will reduce the risk and severity of those injuries.