A lot of healing is about instinct and reflex. While some damage is more predictable than other kinds, in general, raid healing at least is very reactive. When you rely on reflex to do your job properly, that means you have to develop certain habits, and train your fingers to respond without requiring too much forethought.
Of course, the problem with this, is that it is very easy to form *bad* habits. This is one of the things that makes it difficult to switch from Holy healing to Disc healing by the way... the styles are so different, though most of the spells are the same. What is a good response and a good habit for one spec is a truly bad idea for the other. For instance, if I'm in the habit of bubbling raid members when I have some down time as a Disc Priest, that's great. However, bringing that habit over to the Holy side is not nearly as great an idea.
So, in order to get us all in the proper mindset to think about our habits critically, I'll start by confessing all those nasty dirty healer habits *I* have.
Circle of Healing. It's just so easy! It's instant, it's powerful, and I already use it so much, which is a good thing... that sometimes I just can't make myself stop. Everyone is at full health, and I see one person at 95% health, so what do I do? I cast CoH. Go ahead, laugh at me, I deserve it. I use CoH improperly and inefficiently all the time, because it is instant. If I see someone take damage and they need some healing right away, I have a bad habit of just CoH on them to get them some instant healing... even if there aren't 4 or 5 other people who need the heals.
Prayer of Mending. If you are specced into it, the cooldown on casting this spell is much shorter than how long it can spend bouncing around. Granted, if it has "settled" on someone who doesn't need it, it IS a good idea to start it bouncing on the tank again. However, if PoM is still bouncing around, why cast it again if it's not needed? I have just wasted the charges that remain on my previous PoM.
Renew. Because I'm specced into Empowered Renew, this HoT gets a nice little instant upfront healing portion to it as well. Sometimes, if I see that someone needs healing, I will just cast renew on them in an effort to take advantage of the initial upfront healing. In fact, sometimes I've even been guilty of casting Renew on someone once... twice... three times, right after one another, just to get the instant heals on them. Baaaaaad... that brings us to the next one...
Healing through HoTs. I know I am not the only one who has this habit. Someone has a HoT on them, whether it be from one of your own Renews, or from a nearby Druid. In some situations, it makes perfect sense to cast another heal on the Hotted person, or they may take too much damage before the HoT finishes its heal - particularly on tanks. But, if someone in the raid takes some one time damage, and someone throws a HoT on them, don't Flash Heal them... let the HoT run its course, or you are wasting someone else's mana.
Staring at Healing Bars. This isn't entirely our fault... part of this is simply due to how the Blizzard UI handles heals. I don't know a solution for it, mind you, but as a healer, how many fights have you actually *seen*? Most of the time, I have no idea what's going on in the fight, really, because I'm too busy staring at health bars. This lends tons of problems to situational awareness. Half the time I may not even notice my own health is low. But where this really sucks? Fights like Hodir where you need to look for soemthing around you to stand near it for a buff. I don't have time to look for a toasty fire! Mages are DYING ALL AROUND ME.
Forgetting to Heal while in FailAngel Form. I know, sad isn't it? Why do I even spec for it if I'm not going to take advantage of it? But a lot of the time I'm so distracted by the fact that I just died, that I completely forget to, you know, use my free heals to save the group from a wipe.
Not Bubbling Enough. This is only a problem for me when I run Disc... and I think it is mainly a problem because I normally run holy. Regardless, when i run Disc, I just cannot seem to bubble *enough*. I underestimate my bubbles, and I run out of mana, because I'm simply not shielding the way I should.
Neglecting my Cooldowns. This applies for trinkets that have to be popped, but since I know this is a weakness I have, I usually avoid those trinkets anyway. No, the biggest problem are spells like Divine Hymn and Inner Focus. I don't spec into Inner Focus for a very good reason - I always forget to use it. Divine Hymn is a truly amazing spell... that I never remember to use. It has such a long cooldown, I just cant seem to get myself into the habit of using it. My goal is to use it during every boss fight for the next month, even if it's not needed, just to remind myself that it is there, and to get a handle on how powerful it is.
Not using Binding Heal. This is not a problem I have, because I identified it in myself, and fixed it, but I see a lot of other Priests who do not use this spell nearly enough. There is little reason to ever simply Flash Heal on yourself. You might as well Binding Heal to someone else who has taken damage, or likely will while you are casting it. Binding Heal is your friend, priests! It is low threat, it builds stacks of Serendipity for Holy, it does all the fun procs... no, it is not the most efficient heal ever, but it keeps you from having to potentially sacrifice others while keeping yourself alive. If you are Disc, don't Penance on yourself! Penance on the tank, or another target, and Binding Heal if you have taken damage. Don't waste those potential juicy procs on yourself, since it is unlikely you will be taking the most damage.
One of the best ways to improve as a healer is to identify the bad habits you have, and try to fix them. So what about you? Be honest with yourself! What are some of your worst healing habits? Do you share any of my bad habits? I didn't even mention all my keyboard turning...