a large portion of that is knowing the fights (understatement alert)
if a person takes 99% of their health in 1 hit most sane people will land some pretty high hps spells on them to top them (shield + fheal for example). however if they aren't going to take damage for the next 30sec then you can probably afford to take your time.
being a good healer is all about knowing the fights (and the people you play with to an extent). predicting the incoming damage is important for every healer, not just disc.
as is working with your other healers. do you need to top those people up or does that rdruid probably have it covered?
spend time learning how much each of your spells heal for, how much they cost and how they interact. Is 3x PoH on a group with 3 people injured more effective that 3x gheal?
that combined with smart CD timing and knowing your gear (do you have more mp5 than needed for a fight? can you spare some mana on shielding more people? etc)
it all adds up.
practise, and logical thinking will get you 99% of the way.