I had mana issues the first few times trying the first boss in throne of tides. Priest (disc) is my healer.
A couple things were the cause.
My team was taking too much dmg, so was going out of mana keep them all up. 1/2 way through the fight was out of mana, then of course everyone died :). Solution to that was, concentrate more on avoiding dmg than doing dps.
Second thing as was mentioned above, Flash heal, I was casting this as my preferred heal. Since then I have stopped using it unless I need to, to keep someone from dying.
A third thing I got set up to do but haven't needed, is emergency healing with my shaman (they never run out of mana so figured a few chain heals might save the priest some mana, haven't really needed yet though).
Those two things made mana a non issue really. Haven't had mana issues since really. (granted there are more instances I have yet to try (only lvl 83)
The one other thing I did, which helped some I'm sure is I bought a few pieces of gear for my tank, I was in i232-i245 mostly on my tank, so bought greens at lvl 81/82 to fill out some spots that I hadn't filled with quests/random drops. Oh I also ran blackrock depths several times and got a few upgrades there. Boosted my HP a lot which made healing less of an OMG i'm about to die and more of a steady state thing.
