I thought I'd start posting tips for Disc priests in Icecrown Citadel. Let's start with Lord
Bonesaw Marrowgar himself! Marrowgar is a basic fight, and the healing requirements are pretty bare-bones (pun intended). But for each of his abilities there's something you can do to maximize your survival.
His Saber Lash is the first thing to deal with. Normally there will be healers on each tank. You can help them all by shielding each tank as often as you can - the shields will definitely get consumed. If there is another Disc priest in the raid, tell him that you have no idea how those Weakened Souls got there, maybe his spellbook is leaking? Another, less obvious trick is to use Prayer of Mending on cooldown here. With the splashed damage, it will be a big part of your tank healing.
The Bone Spike Graveyard is the other major spike damage (pun intended). I strongly recommend configuring your UI or AddOns so that you can clearly see the debuff this puts on targets, so you can immediately shield them, even if you're on tank duty. This will give other healers time to acquire those who need healing. You can also toss a Penance or Renew their way as needed.
There are also Lightning Tendrils to deal with. I can't wait to see these one-shot people in heroic mode, but for now they're not too bad. A Chain Heal or PoM splash will usually help people survive them. If you try to heal all the people standing in the fire, your mana pool will go bone-dry. (pun intended) (ok I'll stop)
Finally, everyone's favorite whirlwind, BONESTORM!!! What I like to do here is a little tactic called "Shield Spam Panic." I'll let you deduce the subtleties of it but just try to toss out shields (and PoM again) as needed. Although if people spread out to too wide a radius to heal, it won't be very humerus. (who was I kidding)
POWER INFUSION TIME
The best time to cast PI is right after a Bone Spike Graveyard ends. If you do it during or before a graveyard, it may be wasted if the DPS is re-acquiring targets. If you do it right after a BONESTORM!!! (threat wipe), the DPS may get drunk with power and pull agro. This may cause hilarity but not generally success.
BONUS STRATEGY HAIKU
Saber Lash hurts bad
So don't help "tank" Marrowgar
And you won't get boned
So there's Marrowgar. The "tricks" I've posted here are not exactly rocket surgery, but hey he's the first boss. I hope to post more on the following bosses. Till then!