Like I mentioned in part 1 addons are mainly there to help you speed things up. And like I mentioned here, addons won't do anything that isn't already doable in the game, they won't actually add anything. They will however present information and allow you to use your skills in a way more suited to your likings. In part 1 I talked about overview and information. Here I'd like to talk about the second part of healing, and maybe the more important one actually - the ability to react within a split seconds notion.
Having information fed to you in an optimal way (according to you), is of course a huge part of this. The question is then, how will you then use this information as fast as possible? There are some things that will slow down your reactions, some of these you'll simply need experience to "get rid" off. Such things are to choose which heal best suits the situation (unfortunately, an addon that suggests heals at all times doesn't work as well as those addons that suggest skills for dps'ers), and who needs it the most. These decisions you need to be able to make for yourself, if you want to be the best healer everz.
Once you've made that decision however, there are some addons that can help you carry them through in the best (i.e fastest) way possible. Some years ago there was an addon named Healbot. It could actually both choose target and proper healspell for you, which means you basically just had to hit one button to properly heal an entire raid. That addon was probably more efficient than many healers out there. Blizzard thought this was making things a little too easy and changed the way addons can interact with things in the game. Addons like Healbot don't exist anymore, to my knowledge, or at least they don't work in the game anymore.
One of the features of Healbot, not having to target a specific person to be able to heal them, is still doable in the game, and still as awesome for quick reactions. One addon that does this is Clique.
It is difficult to describe what Clique does without having people say "well isn't that what I'm doing already?". The answer to this of course is "no". There are two standard ways to heal a target. A your either pick your target and then what heal to cast on it or B you pick what heal to cast and then your target. What Clique does is letting you choose both heal and target at the same time, which might not sound like much, but will really make a huge difference once you get used to it. Clique offers a really easy-to-understand-and-use setup for you to adjust which bindings to use and then you're ready for some awesome healing action!
I myself have some different bindings depending on spec and so. My favorite is binding my mainheal (renew or shield depending on being holy or disc) only to mouse right-click button. This means I don't even have to use the keyboard to cast this skill on someone and can use all the fingers on my left hand to run around avoiding things instead, while casting these particular spells.
The best thing to do is try some different settings and see what fits best for ones playstyle. I for one keep spells I don't cast as often on shift, and the ones I use more often on alt, since I press shift with my little finger and that tends to hurt alot after some hours of raiding ^^ Little things like this you will notice and be able to optimize after some playing so try it out!