For starters: know what you're working with
Just to be super 101 about this (because I think this is a really important topic), keep in mind when it comes to active mitigation, we have two types of abilities: holy power generators (HPGs) and (for lack of a better term) holy power spenders. Our generators are Crusader Strike, Judgment, and Grand Crusader procs. Our spenders are Word of Glory (or Eternal Flame, if you talent for it) and Shield of the Righteous.
Shield of the Righteous, when it comes to spending holy power, is our bread and butter. The grand majority of holy power you spend will be on this ability, since it's what builds up our Bastion of Glory buff, and allows us a guaranteed "block" for any physical attacks taken in the next three seconds. The execution of ShoR is the cornerstone of our survivability, and the more often you can get it out, the better (with an important corollary I'll describe in a bit).
Word of Glory is different. Unlike most of Cataclysm, Word of Glory does not have an appreciable cooldown anymore, so you don't have to sit and hope and pray for it to become available when you need it most. However, thanks to Bastion of Glory, you don't want to use it that often anyway.
Maximizing your holy power generation
Like a car engine, your active mitigation won't take you very far unless you put enough fuel in it. Every HPG ability you use is more active mitigation you can throw at any incoming damage. As such, it needs to be your absolute first priority to keep the engine running: the holy power must flow. When deciding which abilities you are going to use in the course of a boss fight, you always want to use a holy power generation ability first. Crusader Strike is priority number one, then Judgment, then Grand Crusader. Keep in mind that the Grand Crusader buff lasts 6 seconds, so you have four GCDs to get it off; no need to panic if it pops when CS and Judgment are available.
If you delay using an HPG to drop a Consecration or throw down a Light's Hammer, you're ultimately going to be pushing back your active mitigation, which will hurt your survivability in the end. This is the biggest mistake you can make in your rotation!
Every global cooldown is crucial. It's zero-sum in a way: you spend it, or you lose it. If you delay in using an ability, that is also pushing back your holy power generation and your active mitigation. And thus your survivability. I've written about this in the past, but one of the biggest secrets to maximizing your output is to hit more buttons -- that is, don't let GCDs sit idle, and don't ever, ever, ever hesitate using your of your HPGs. The bright dividing line between the bad tanks and the good tanks is how often they're hitting their abilities.
Actively mitigating to great effect
For starters, allow me for the hundredth time to sing the praises of banking holy power. That is, you want to only use a holy power spending ability when you're at five holy power, so you're reserving at least two at all times for emergencies.
Likewise, it's perfectly kosher to spend the only three holy power you have on a Shield of the Righteous or a Word of Glory if you're in some degree of a crisis, be it at a low health percent or a healer is out of action, or what have you. The idea is just to be more like an ant than a grasshopper when it comes to your holy power.
Now, as I alluded to above, there is definitely a benefit to being very Spock-like in how you use your Shield of the Righteous. With a three second window on the "block" you'll want to get the most return on your holy power investment, so if you can line that up with a very imminent and very dangerous physical attack, then you'll be in a good place.
However, this is the kind of play that heroic-level tanks will be working on, so for the rest of us, it's not anywhere near hurting one's self just using your Shield of the Righteous as per normal. (This is all just something to keep in mind to give yourself an edge, if you want to give it a shot.)
Word of Glory, moreover, should be approached as if it were a cooldown, used only at a critical moment to offset a big hit, or buy your healer some time to top you off after being knocked to a very low health level. And the higher your Bastion of Glory is when you do use it, the better!
There's no real value to constantly spending your holy power on Word of Glory, as you'll be depriving yourself of those critical "blocks" and denying yourself Bastion of Glory stacks and thus weakening your WoGs -- it's going to hurt your survivability to follow that path.
Never forget that now more than ever, you control your fate and your survivability. The single greatest determinant between emerging triumphant atop the felled corpse of your monstrous foe, and being ground beneath its boot, is how you utilize your active mitigation toolbox. Well, and the healers help a little too, I guess.
The Light and How to Swing It shows paladin tanks how to take on the dark times brought by Cataclysm. Try out our four tips for upping your combat table coverage, find out how to increase threat without sacrificing survivability, and learn how to manage the latest version of Holy Shield.