I am pretty familiar with black bears. Not so much grizzles, and their behavior is quite different.
#1 rule of black bears is they feel the same way about you as you do about them. if you come around a corner and BOOM you are within swatting distance of a bear and he is suprised, you might be in trouble. thats the idea of bear bells, to give some advance warning. a dog or walking companions or singing also works.
When I see a bear, I stop, check for cubs, stay in plain view, and slowly get closer until it notices me. then i stop, take pictures and wait for it to either back off or stand its ground. keep eye contact, move slowly and deliberately. i have never had a bear do anything other than evaluate the situation and either tolerate me or leave.
Worst comes to worse and you get in a bad situation, prevailing wisdom is first back away slowly maintaining eye contact, and if you get charged (they paw the ground and bluff a few times first), either play dead or fight back while yelling, depending on how commited you think the bear is. I know of several people who have been charged and their yelling and stone throwing and stick swinging made the bear change its mind. Once the bear makes contact, play dead, they arent hunting you for food, they are trying to win a fight, so make them think they won. Never EVER EVER climb a tree or try and run away, bears legs are twice as long as they look, and they dont slow down AT ALL when they move from flat ground to straight up a tree.
Good stuff, however everything I've ever read on bear encounters states to avoid making eye contact, as it can be perceived as a threat by the bear. Also, when it comes to black bears, you should fight back, instead of playing dead.