Don't give up- often much can be done in that fraction of a second to make the best of it.
Tuck and roll- spread the force, don't take it all with the arms, protect the head.
Ride defensively, be aware of potential hazards, outlets, and landing zones.
Wear gloves. Some slow trials-type riding is good for bike handling & can improve emergency maneuvers.
I knew a guy who practiced riding increasingly fast into crashes on a lawn, but he died of a degenerative disease so there's that...