Spamming shield was fine before the mana pool nerf. I could have shield as 50% of my healing with only 2,200 spirit(no trinket) and be fine on every heroic fight but spine(equiped the spirit trinket for that one). Now that my mana pool has gone from 175k to 100k, I've had to reforge a bit and alter my play style.
Also mastery benefits divine aegis(100% chance with prayer of healing) and spirit shell.