ABC of healers and casters: Always Be Casting
If you have nothing to heal > PewPew
Use your MP regen on cooldown

Start small, ensure your dots are on boss 100% on the time
When you have access to Holy, give at least one on each pulls (Stunned mobs = no damages)
Avoid using Cure 1, prefer oGCD heals when you have them. Cast heals only when you have nothing else to heal with.

You will soon realize you dont need to heal as much as you think. People dont need to be topped off all the time
Plan when you use Benediction, the best single heal in the game (and free!). Let the tank lose damage, use it before it is too late.