I don't know about all bear spray, but the Counter Assault I have comes with a nylon holder/holster with velcro that easily fits onto a belt. I recommend everyone buy a "test" cannister. Discharge it. Test fire it. Be familiar with how it works. How to get the safety off. Where the spray goes and how far. Fire it off in the rain or snow. In the wind etc. Know what it will do. Would you really walk into a firefight with a weapon you'd never ever fired before?
There's a bunch of situational answers to this. Black Bear or Grizzly? What is the bear's posture? And others. But generally you want to A) Let the bear know you are human and not a threat. Talk, don't shout, at him. B) Move away in a subordinate manner but don't present your full back to him. C) Do NOT run. Crap your pants, shake, but do not run unless you KNOW for certain you can get to your car, cabin, or a safe place before the bear reacts and gets to you. No race horse can out run a grizzly bear, so unless you have a big "S" on your chest, don't run.
After looking at the threads about carrying firearms and bear spray, I felt the need to clarify a few things.
1. Some think the threat of bears is exaggerated. Maybe so. I'm not sure what constitutes exaggerated. I certainly know that both black and brown bears are not predatory carnivores hiding behind every tree waiting to pounce on innocent humans. This thread was mostly intended to be about YNP bears which are, at least to some degree, habituated. That makes the threat level go way up. The YNP bears push my alert ears into high gear. Way more than any where else. When I see tourons in YNP walking toward a grizzly sow and cubs to get a photo, I see an idiot who doesn't have a clue. I also see a dead bear. The way I see it, the further you get from people, the less the bear threat becomes. When I was in the bush of Alaska I didn't worry much about bears, but I did keep my eyes, ears, and nose open. When bears wanted the dead salmon at my feet, I was happy to give way and let them have it.
2. Bear spray. I've encountered both black and grizzly bears where I've seen them far away. I've also encountered them when they were very close and were moving very fast. In the first case you'd have plenty of time to get your bear spray out. In the second case you'd have to rely on what you learned from doing research as to how to act when face to face with a bear. The response would depend on several factors. That was my point for the thread - do your homework before you go into bear country in YNP.
Bear spray is not a substitute for good sense and knowledge. And practice. Mark mentioned how he and his buddy randomly practice deploying their bear spray. That's friggin' brilliant. They are prepared for something they really, really, hope never happens. It's prudent. If you buy bear spray and don't even take it out and practice deploying it, it's worse than having nothing at all because you may stop being aware and just rely on the pepper spray to save your ass. Not smart.
Your best defense is your human brain. I've avoided conflicts with bears because I was aware of the potential hazard before it was on top of me.
Bear spray isn't for every situation. I've wrangled for dude ranches where the lead wrangler, would, before the riders mounted up, hold up a cannister of bear spray and ask "Who wisely brought they bear spray with them?" Hands would go up. He'd then say "Keep your hands up so the wranglers can collect them. You can get them back after the ride." A scared out of his mind rider deploying pepper spray from the saddle at a bear is going to turn a frightened horse into a blinded and panicked horse and a rider on the ground.
3. Firearms. I spend a lot of time around horses. On occasion we lose a horse or have to put one down. We lost one last fall. A crippled horse is an awful sight. They have very small brains with thick skulls which makes putting a bullet in their brain which is the size of a racquet ball, difficult anyway. If you ever saw someone trying to put his horse out of it's misery by bashing it's head with a rock as the horse thrashed around, you'd know the value of a firearm in the backcountry even if you had no fear of bears, tweekers, etc.
Few of you are going to end up doing what I do, so the choice is yours to carry or not carry. That's the bottom line in my world - choice. To choose one way and condemn anyone who chooses differently doesn't make any sense to me.