I've struggled with handling the number of abilities issue, especially with some of the more subtle abilities (hand of salvation, etc). My personal technique for handling this, is to simple focus on a new ability for a few runs. I'll do a postmortem of a past run and notice for example, that I'm not using Divine Protection. Obviously not using an ability that mitigates damage and therefore reduces healing is big no-no. This ability is especially valuable when used in conjunction with Divine Sacrifice, since you can redirect damage a player that is about to get slammed and mitigate it. Using the Glyph of Divine Protection makes it especially powerful when handling big damage spell based abilities. After this research, I make a new macro:
#showtooltip Divine Sacrifice
/cast Divine Protection
/cast [@mouseover, help] Divine Sacrifice
I then link it to a keybind and focus on using that ability in the next few runs. During these runs, I'll be thinking about it, as I let my normal healing routine run on automatic. After a few runs, I won't have to "focus" on it as much, and eventually, it'll just be part of my arsenal. This has a lot to do with learning each encounter and anticipating instead of reacting with an ability. I know the big bad Creep is going to blast the tank with a Giant Ball of Fire soon, so I can am ready with Divine Sacrifice. Once it's in your head as an option, it'll be more natural to react with it too.
I remember on my casual Holy Priest, I'd run several heroic 5-mans, without ever using Guardian Spirit. A 31-point talent with a short 3-min CD that I never used is shameful. I even had a key bind on it. But, now I pop that GS on players all the time, especially in heroics, when running with a Tank that doesn't like to CC.
Like all things worth doing, the room to improve is key to making me come back to the game raid after raid.