I can not really speak for holy as I mainly play as discipline, but as a priest it is really all about managing your shadowfiend, hymn of hope and trinket cooldowns (+arcane torrent if you're a blood elf). I recently killed Cho'gall while duo healing as discipline together with a resto druid, and the kill was insanely taxing on mana - only with perfect timing on shadowfiend did I manage to be able to nearly chaincast heals the full 8 minutes of the encounter.
In case you're interested in my playstyle, I frapsed the encounter and you can find it here http://www.youtube.com/watch?v=FJdxXhQH3-E