Reaching the HPS cap
Every time a tank dies, a holy paladin learns a little more about himself. I tend to view every tank death as a personal failure. The most common cause of tank death, in my experience, is doing too little healing per second. I'll get complacent while using my cheaper heals, and before I can react to a change, the tank gets smashed.
When you're healing a tank, you're essentially playing tug-of-war with the boss. The boss is doing a certain amount of DPS to your tank, and you need to do a certain amount of HPS to counter that. The boss might change his DPS values up and down at different times to try to throw you off balance, so you need to counter with the proper HPS choices quickly. The tank's life becomes a buffer, giving you a few precious seconds to react to the boss' latest attacks.
If the boss is doing more DPS than you're doing HPS, you have three choices: lower the boss' DPS, increase your personal HPS, or ask for some help. Asking for help should be our last resort, so let's focus on the first two options.
In order to reduce the boss' DPS, the tank (or raid) can use some of their cooldowns to mitigate some of the damage. Every tank has a couple of cooldowns at his disposal that help drop incoming DPS, and so you should coordinate with your tank to use those cooldowns if you're finding that you don't have the HPS to keep up. We can also use our defensive cooldowns, like Aura Mastery and Hand of Sacrifice, to reduce the damage that our targets take. Finally, you can also use CC or interrupts to knock down your enemies' damage, where applicable.
Defense isn't everything
Cooldowns are nice, but they're not always the answer. Sometimes, regardless of what cooldowns are active, spamming Holy Light with every global cooldown won't keep someone up. If you find yourself without enough GCDs, you might just need to use a bigger spell. While Holy Shock and Holy Light are great for saving mana, their HPS won't keep up a tank that's under pressure. There are situations where you just need to start dropping Divine Lights if you're going to keep someone alive through heavy damage. Avoid using Flash of Light if you can, as it takes the HPS/HPM balance too far and will rip through your mana way too quickly. When it comes to holy paladin spell selection, the real choice comes down to Holy Light vs. Divine Light.
The key to choosing between Divine Light and the other, cheaper heals is knowing how much damage your tank is going to take. If it's just a short period of burst, a few Divine Lights won't break your mana bank. If it's going to be a sustained session of spamming DL, you need to make sure you have the mana for it.
On a fight like heroic Halfus, where spamming Divine Light is really the only option, we have to use every trick in our book to prevent ourselves from simply running out of mana. Every gear upgrade we gets goes towards increasing the number of DLs we can cast per fight. DLs are typically the limiting factor of any mana pool, and so you want to spend the ones you can afford wisely. If you do have to spam Divine Light, you can mitigate the amount of mana you spend by utilizing Beacon of Light and Tower of Radiance. If you just have a single target getting slammed, you can toss Beacon on them, and then use your ToR holy power points on free Words of Glory. I use this strategy on several heroic encounters, as the free WoG every few seconds comes in handy for reducing my total mana expenditure.
The final way to increase your HPS is to use an offensive cooldown to boost your healing output. Avenging Wrath and Divine Favor both boost our healing significantly, and our Guardian of the Ancient Kings assists with this, too. Between our defensive and offensive cooldown options, we can tweak the DPS/HPS balance in our favor.
Defensive cooldowns tend to be the most potent when the tank is getting hit for huge amounts, while offensive cooldowns work best when the tank's life is descending smoothly. How much less do you have to heal when a tank has Shield Wall up? You'll never know until you try, so you need to test the potency of each cooldown to know when to use them.
Hitting the mana wall
While it's easy to tell you to just use Divine Light more often if your tank is dying, you also can't sustain a 100% Divine Light cycle. We simply don't have the mana to spam our biggest heal like we did in Wrath. Without careful use of our cheaper heals, we'll run out of mana before an encounter is over. We have to be as efficient as we can without letting anyone die.
Riding the line between doing enough HPS but yet reserving enough mana is one of the major difficulties of playing a healer. In Cataclysm, we don't always have to top everyone off. I see a lot of newer holy paladins pulling out their big heals on every pull, when they can usually get by with far less.
For example, against Bloodlord Mandokir, we don't have really heal anyone besides the tank. Mandokir's debuff, Ritual of Bloodletting, won't kill a player as long as they have a few hundred life, and his power move, Devastating Slam, kills on impact. There's no reason to spend the time or globals on healing the group, since they're in no real danger.
Similarly, when facing Cho'gall, your raid won't take much damage during phase 1. While they'll get blasted during the Shadow's Orders phases, you have a ton of time to heal everyone up. You don't need to be in a rush; you can use Holy Light to get everyone back up to full before the next Shadow's Orders phase. While Divine Light might heal twice as fast as Holy Light does, Holy Light can do it for a fraction of the mana. Lean on Holy Light and Holy Shock whenever someone isn't in any immediate danger of dying.
If you find yourself running out of mana, you should approach it from two directions. First, try to get more mana by using Divine Plea and Judgement as often as you can. Second, try to use cheaper heals whenever you can.
Unfortunately, you will do less healing when you're trying to save mana, and so you run the risk of having someone die because you weren't healing enough. Every once in a while, someone will die and it will be your fault. You are the ultimate dispenser of your healing, and so you must be the vigilant guard that ensures that the heals are doled out appropriately. The choice between Holy Light and Divine Light is not always straightforward, and you will have to trust your instincts.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Learn the ropes in Cataclysm 101 for holy paladins, study the new balance between intellect and spirit and learn how to level your new Sunwalker. Tanking is a job, DPS is a craft -- but healing is truly an art.