Let's start by taking a big picture view of PvP in WoW. In my mind, there are three main types of PvP interactions: small-scale burst, small-scale outlast, large-scale survival. The best example of a small-scale burst fight would be 3v3 or 5v5 arena team, where your focus is to score a quick kill and then clean up your opponents. Small-scale outlast would be more like a 2v2 fight, where your goal is to stay alive and either run your opponents out of mana or catch them making a mistake. Large-scale survival is the battleground and world PvP model, where you're expected to die occasionally and the fights are almost never balanced.
Your gear and strategy choices will depend greatly on which of these environments you plan to focus on. One thing that doesn't change based on environment is the value of resilience, which is the best survival stat in PvP. Make sure you're wearing as much PvP gear as possible, as without it, you won't have enough time to heal yourself if focused on. With only one truly instant heal, we can't afford to be casting spells while being attacked. A PvP trinket is also considered necessary, since you can't heal when you're Cycloned.
The goal of any healer in any environment is to keep their teammates alive, while also keeping themselves alive as well. However, in PvP, due to the incredible amount of variation in encounters and possible outcomes, we should also be on the lookout for any way to assist our team that doesn't necessarily involve healing. If there's a wounded enemy trying to escape, we have several ranged attacks like Judgement, Hammer of Wrath and Holy Shock that we can use to assist with executing a priority target. We also have a few CC options available. Turn Evil can control warlock and death knight pets, while we can also stun any target for six seconds with Hammer of Justice. While keeping your teammates alive should be priority number one, any spare GCDs you have should be used to push the momentum of the battle towards your favor.
In any sort of burst environment, where your goal is to quickly kill an enemy because they can't be resurrected, cooldowns are what will turn the tide of a battle. The fights aren't designed to be lengthy, and so using your CD moves early will ensure that you're not caught dead, wishing you'd used a more powerful ability instead. Hand of Protection and Hand of Freedom are key for ensuring that your casters and melee are uninhibited by the enemy team's focus and CC attempts. You want to put as much pressure as possible on your opponents, so stunning your kill target or one of their healers will hurt their tempo and let you keep the momentum. In addition, any extra damage that you can add via a Holy Shock or Judgement will assist in getting a quick kill. I like to use Aura Mastery with Concentration Aura active to give me a free six seconds of casting where I can't be interrupted or silenced, which also works for any casters on your team as well.
In small-scale burst PvP, you'll want to be using heavily offensive gems and gear, focusing on spellpower and critical strike rating. You can usually gem to meet your socket bonuses in your PvP gear, since having a balance of stats is a good thing. The fight isn't going to last very long, and so mana conservation and regeneration are secondary. Make sure you have Hand of Sacrifice and Divine Sacrifice ready to use, as they'll not only reducing the amount of damage that your own team takes, they'll also allow you to break out of various crowd control effects when any damage is redirected to you. You'll want to make sure you have plenty of resilience if you plan to play in this environment, as if you are focused, you really only have Divine Shield to fall back on once.
While outlast encounters are often quite long and can be boring at some points, they are also highly technical and smart play will make a bigger difference here than in any other PvP situation. Your goal is to use as little mana as possible while making your opponent use as much as possible. Little things like melee pets with Seal of Wisdom active can determine the winner of a match when both healers are running low on mana. Sacred Shield is our key spell in an outlast comp, since its mana cost is very low but it provides a significant amount of absorption, allows our Flash of Lights to crit more often and adds a HoT effect, and can reduce spell pushback while active. Holy Shock is an expensive heal and should only be used when necessary, while Flash of Light and Sacred Shield will do most of the work. Don't waste mana on things like Consecration, as the small amount of damage added is not worth the huge mana cost.
You'll want to focus on mana conservation when you're playing on an outlast team, and intellect and MP5 are definitely the best stats for a holy paladin trying to stretch out their mana pool. MP5 works whether or not we're casting, which makes it ideal for a paladin who is casting pretty often. Intellect helps boost our starting mana pool and percentage-based mana returns like Seal of Wisdom and Divine Plea. Spellpower is also important for boosting our Sacred Shield and FoL potency, and so SP/MP5 for blue sockets and SP/intellect in yellow sockets can work really well.
If you're in a battleground, you should expect to die. It's bound to happen at one point or another, and it's not always your fault. Sometimes your team will send two to the mine while theirs sent five, and that's just how the cookie crumbles. The key is to stay alive as long as possible, harassing whenever possible, and making each death count for something. There's nothing wrong with using your big heals in a BG, since you get all of your mana back after a death anyway. The key to battleground play is figuring out which target is taking the most damage and getting heals to them as soon as possible, while also watching out for your objectives.
Take Arathi Basin, for example. If you're at the blacksmith and you're under assault, you might be forced to choose between healing an ally or defending your flag. If your only chance of surviving the attack is with your teammates' help, then sacrifice the flag to keep them alive, and then take back the flag. If you have no chance of survival, you can let your ally die and stop the capture, and then start running away. Use Judgement or Holy Shock to interrupt any additional attempts to capture the flag, while trying to kite your opponents to the best of your abilities. You're buying your team a few precious seconds of flag control, and delaying several enemy players who could've already been moving on.
I like to use Holy Shock and Holy Light in battlegrounds, with a focus on Holy Shock if I know there is someone trying to interrupt me. You should be saving your cooldowns for situations in which you can turn the tide of a battle with your CD. If you and a warrior are up against a shaman and a rogue, you could run into the fray first, grab the enemy's attention, and then use Divine Shield to negate all of their attacks after they've already invested themselves in focusing on you. You've now guaranteed that your team has won that small encounter. Using Divine Shield to buy yourself a few meaningless seconds of life after your base has already been overrun is pointless and by delaying your death, you've actually set your team back.
I like to focus on spellpower and haste when playing in a battleground, as when a large battle is taking place, you'll want to get as many GCDs of healing done as possible. Look for opportunities where a quick dispel can do the most good, like removing a Polymorph or Fear. Don't waste time dispelling small debuffs like Judgement of Wisdom or Faerie Fire unless you have the free time to do it. Players can die very quickly in a large world PvP or battleground fight, and so healing will usually be your top priority. Watch for any indications that the enemy team may be focusing on you, and have your finger over your Hand or Protection or Divine Shield button.
While it's hard to provide any specifics for healing in PvP due to the sheer number of possible actions by both teams, there are a few key ideas that can guide your decision-making process. Triage your party constantly, focusing on those who are closest to death and using any CDs possible to handle situations where your teammates are dying faster than you can heal them. We have a lot of available buttons to push to assist our team, with three of the "Hand of" spells being incredibly valuable in PvP. Holy Shock is typically your best friend, and using the Glyph of Holy Shock is almost mandatory in PvP. It's instant cast, which means you can't be interrupted while casting it, and it opens up the possibility of a second instant heal via Infusion of Light. Lastly, don't hold onto any CD for too long. Use them any time that they'll see a significant benefit, since dying and having all of your CDs available sucks.
The Light and How to Swing It (Holy Edition) is dedicated to helping holy paladins become the powerful healers that we're destined to be. If you're new to the paladin's healing ways, you can learn the ropes with our Holy 101 article. We also have information on how to keep a tank alive, how to heal a raid when necessary and how to beat the GCD. Tanking is a job, DPS is a craft -- but healing is truly an art.