First of all, it's going to take some significant haste to even get your holy paladin all the way up to the Haste Soft Cap (HSC). This is the point where your 1.5-second spells become 1.0-second spells and where your GCD goes from 1.5 seconds to 1.0 second, as well. That's as low as the GCD can go, which means that past this point, you're limited to casting a spell every 1.0 second and no faster. Increasing your haste past the HSC will make your longer casts faster (like Holy Light) but will not help any of your other abilities.
Haste past the HSC is worthless for a FoL paladin and becomes slightly less valuable for HL paladins. Even with this in mind, it still beats critical strike rating into the ground in terms of HPS benefits, and so HL paladins will be picking up haste well past the HSC. For a holy paladin with 5/5 Judgements of the Pure active and the haste buffs of Wrath of Air Totem and Swift Retribution, 676 is the Haste Soft Cap. Getting to this number is pretty important, as most of our abilities activate the GCD, and hitting the HSC means more actions per minute for us no matter what we're doing.
Off the GCD
One thing that can help us get around the GCD limit of 60 abilities per minute are abilities that don't activate the GCD at all. We have an assortment of spells that can be used instantly or chained together with other abilities. Our most versatile defensive ability, Aura Mastery, isn't on the GCD and can be cast without losing our Holy Light pacing. Two of our offensive CDs, Divine Favor and Divine Illumination, are also both off the GCD for quick usage. I like to tie Divine Illumination to Divine Plea though, with a macro that uses DI and DP at the same time for the cost of only one GCD. Some player-specific abilities are also off the GCD, like Arcane Torrent and many of our On Use trinket actions.
To maximize the usage of these non-GCD spells, you should use macros to activate them at the same time as a GCD-based spell, so that you don't have a delay between pushing button 1 and button 2. By reducing this player lag between abilities, you can save yourself even more time when using these non-GCD spells. I like to macro Aura Mastery to a Holy Light, so that I can start spamming that macro when I want AM to be used and yet still not even lose an instant of Holy Light casting time. This is especially important on fights where you can't afford to stop healing to use the spell. As a rule, if you have a non-GCD ability, try to macro it to a GCD ability to save yourself time every time you use it!
I've been dying to use that word in an article since I watched Minority Report again a few months ago, and now I finally have the chance! A key to getting the most out of our one-spell-per-second restriction is to synchronize our GCD with enemy abilities. A great example is Decimate: it's a widely telegraphed attack that will do a predictably high amount of damage to a predictable group of targets. What we can do to counter the spell is to time our next Holy Light to land immediately after the Decimate goes off. This means we get a "free" cast in, instead of waiting for Decimate to strike and then healing.
Healing through abilities like Soul Reaper on the Lich King encounter pretty much require precog healing, as there isn't enough time to see the damage land and get a Holy Light to the target before they die. Other healers can use their preemptive heals and HoTs to handle these situations, which means they'll be sniping a lot of these proactive heals from us. However, if we're playing at our top, we can literally save lives by having a well-timed Holy Light casting before the target even takes damage. Pre-casting heals is a key determining factor between a good healer and a great healer.
Pace your heals
The final and probably most important thing you can do to work within your GDC limit is to realize that you're capable of one heal per second, and plan your heals a second in advance. Using a healing communication mod can assist with this, and there's one built into most raid healing unit frames. You'll be able to see if another healer has a heal incoming to a player already. Find a player who is low on life and doesn't have a heal incoming, and plan to heal them as soon as your current heal is finished. Now that you have that player covered, plan your next heal. If you're constantly looking ahead for the next player to heal, you can minimize your time between heals, which will ensure you reach your full capacity.
Make the most of every GCD
There are a lot of strategies and best practices you can follow to make sure you're getting the most out of every GCD, though remember that you don't need to be 100% active for every second of an encounter. The trick is to ensure that during the times you do need to maximize your healing, you're not wasting GCDs on non-essential spells or adding delays between casts due to slow target selection or pushing non-GCD spells separately. Make the most out of your 60 actions per minute, and you can be certain that you're giving your all.
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 or how to heal a raid when necessary. Tanking is a job, DPS is a craft -- but healing is truly an art.