Bear Hug: A man once escaped a bear attack when he locked his arms and legs around the bear in a reverse bear hug, which made it confused and it ran off. If you're on your back and the bear is on you, it might be your last resort.
Play Dead: Okay, this doesn't work when a bear has stalked and attacked you, but will work with bears protecting their cubs or territory. So if you are a lover, not a fighter, play dead.
Pepper Spray: Mace, pepper spray, wildlife repellent... they are all great at making a bear retreat. If you don't have any with you, read the other steps listed here.
Eye Gouge: Big bear has you down and is seconds away from having you for dinner. If you have a free hand left, drive a finger into the eye. Blindness ends all excursions.
No Hiding: This seems counterintuitive to staying alive, but hiding will also wake the killer instinct in a bear. Especially stay out of tents or trees. That's practically daring them to come after you.
Calm The Charge: While it may be the last thing on your mind, calmly standing your ground when a bear is charging can be the best attack deflector. If you get a bluff attack as a warning, stand, wave your hands, and talk calmly to the bear.
Go Sideways: Because of the bear's huge upper body and neck muscles, rotating their heads side-to-side is difficult. Keep moving to the side to make it harder for them to see you... or see your attack on them.