For my disc priest, flash heal is actually more healing per second and more healing per mana than greater heal, so I never cast greater heal.
If my tank is taking lots of damage, I make sure to keep up PoM PWshield and renew while spamming flash heal the rest of the time, i look at the tank's PWshield debuff to know when to hit the castsequence for those 3 spells again.

Glyphed Holy Nova keeps me covered for aoe heals if my team is close together, and PoH if they are spread out.