I see so many macros out there combining as many defensive cooldowns as possible into a single button. The benefit being when something goes wrong you probably know right where that button is and hit it without thinking, but the benefit ends there. These are the constructs of leveling/inexperienced players. I think I just insulted a few people, but I can explain.
When you are leveling you don't have anyone to heal you (aside from yourself) so if something goes wrong chances are you aren't in a tanking spec or tanking gear and don't have the luxury of time to decide how many cooldowns you really need and if they could of been used better.
On the other hand you have inexperienced players, tanks for the most part that use these types of macros as a crutch. On one hand it it one less thing to worry about (managing defensive cooldowns), but on the other hand you are glossing over a fairly important part of tanking which will only hurt you in the long run. Especially you death knights out there, your class is built around defensive cooldowns.
So if you are a squishy DPS/Healer type then an oh shit button might be a good tool, but not great. Tanks however should avoid such things like the plague and here is why. If you blow all your cooldowns at once and take near zero damage for 20 seconds then you better hope whatever is hitting you falls over in those 20 seconds or else you go down in that 21st second followed by everyone else in seconds 22 and 23.
If you have a healer and/or a health pool large enough to draw on for a while then the best course of action is to blow one cooldown, look how things are going, and blow another if necessary. Spread them out so instead of 20 seconds of god mode you have 40 or even 60 seconds of decreased damage that will hurt, but won't kill you outright. Healers have cooldowns too and will probably be looking at your health bar to determine when to use them. If you drop down and blow all your cooldowns chances are your healer used one or more as well.
There we have a second benefit to spreading out cooldowns, if you keep taking damage, but a decreased amount, your healer may just be able to keep up without a cooldown so when you run out they might still have a trick or two up their sleeves to keep you alive for another minute and before you know it you have more cooldowns ready to use again.
Also healers like consistency with their tanks, we don't like to see tanks go from taking no damage to taking a ton of damage. Non tanks tend to require immediate attention when their health starts to drop while tanks can generally wait a second or two for you to get a heal off on someone else. As I healer I generally get a feel for what my tank can handle and decide what I can get away with based on that. If something horrible happens and the tank blows everything the healers might use their cooldowns on someone else and not be prepared if the tank starts to plummet soon after or blow all their cooldowns on the tank. The end result being that no one has any cooldowns left.
Another way to be consistent is to use cooldowns preemptively. You certainly don't want to use them all at once in this case and some are better suited to certain situations. For example something enrages for a few seconds and you know it is going to happen before hand, you can have a damage reduction cooldown ready to cancel out the increased damage so it appears to the healer than nothing changed. Then if your healer does notice a change, most likely because you ran out of tricks, they will know that they should bring out the big guns. It will ideally be a small change that tips off the healer.
So bears, we've got Survival Instincts, Frenzied Regeneration, Barkskin, Trinkets, and your consumables (potions and healthstones). Also a couple other things like Bash and backpedaling can be used defensively. Bash doesn't work on bosses aside from an interrupt, but it will stun any trash or adds some of which can do a bit of damage if left alone. Never underestimate running away either; chances are with all the debuffs players put up these days that whatever you are hitting isn't all that fast and you can stay ahead of it while running backwards (infected wounds does the job if you took the talent, but I didn't).
Just a reminder never point your butt at something that wants to kill you as you cannot dodge from the back. There are exceptions of course if you know you will stay ahead and not be hit or that whatever is behind you won't be there long. For example I do a lot of gauntlets back asswards for a couple reasons. I move slowly and consistently along so everyone can keep up and I have an excellent viewpoint to confirm everyone is caught up and make sure they don't have any friends. In a gauntlet you get a few mobs coming at you at a time and while you might be showing them your tushie chances are they will be more interested in your healer. Swiping them just as they are passing has little chance of having one hit you in the back, but even if one or two do you are moving backwards and they won't stay there.
Back to the real bear defensive cooldowns. Barkskin is your first line of defense, if something bad is happening don't hesitate to use it. It has a minute cooldown, even shorter with a T9 set bonus, and is available pretty much every time you want to use it.
After that are trinkets. They vary in effect, some absorb damage, some increase avoidance or mitigation and others increase or replenish health. Most all are on two minute cooldowns. Most of them you can use as a backup or a compliment to barkskin, but if you are taking a lot of spell damage, more armor isn't going to help. I'll mention trinkets that affect your health more below.
When really horrible things happen we have the combination of Survival Instincts and Frenzied Regeneration to fall back on. I generally start with Survival Instincts and if I still see I need more I will go ahead and use Frenzied Regeneration. Frenzied Regeneration heals based on your maximum health so Survival Instincts and any trinkets that increase your maximum health compliment it wonderfully. A lot of times Survival Instincts on its own will be enough to get out of a sticky situation and your healer can use the extra large health pool to catch up. That frees up frenzies regeneration for another time since it is a waste to use it if it would be only over healing anyway and would be better used even without the bonus.
When I have a maximum health pool increasing trinket I will either use it at the same time as Survival Instincts and Frenzied Regeneration or save it for a Frenzied Regeneration without Survival Instincts.
The absolute last line of defense for a tank is health potions, healthstones and any trinkets that provide health. Healing potions don't heal for very much, compared to what a healer can do they are a drop in a bucket for a tank, but you only need a drop to not be dead. I use these types of things when I am really low and one or two hits could kill me. It is basically a little something to give the healer just a little more time to get a cast off. A few thousand more health is not going to save you all on it's own unless you are really close to killing whatever it is that is hitting you. A health potion without fear of imminent death is just a waste.
There you have it, leave your cooldowns on separate buttons and learn to use them to their full potential. You will be a better player for it.