Couple rambling points from Bode!~ Basically tiny modifications and ways of setting up to help avoid AOE.
1- /script SetCVar("cameraDistanceMax",30). This macro allows you to scroll your camera to a distance further than regular options in Blizzard UI allow for, don't worry its legal though. Always raid at max camera distance. Scroll back out if you die, and it resets to normal view. Having a birds eye view of incoming AOE is a key part of prevention. 2nd part is positioning camera to anticipate damage. For example turning camera angle so you have a view of sides on Sartharion so you can predict incoming waves without needing a raid leader to call it out for you.
2 - Buff/debuff bars, if you are a healer this should be right next to your heal ui/raid frames. Classtimers or Elk debuff bar should be fine. Class timers is a good way for watching Beacon/Sacred Shield up time either onyourself or your tank.
3 - Grid + Clique. If you are new to healing or at entry level raiding I would recommend Healbot. If you are looking to take the next step and go to a higher level of raid healing Grid + Clique. Here is a good overview of Grid and why it is such an enormously powerful tool by a Paladin in a top 100 guild.
4 - If you are a healer, put Grid reasonably close to your character!!! Because if you are staring at a corner of your screen and missing AOE you are bad, the less your eyes are darting to different corners of your screen the better, I don't know about you but I am on a 22inc screen and if I had my DBM in one corner, grid in another and was watching my toon at the same time I would be Walleyed and making mistakes.
5 - Click on my keylogger! - I keed, I keed. This is a ret pally, so who cares that his Grid is in the bottom corner (though even if you are dps you should be sporting raid frames at all times). Notice the timer to the side of it and more importantly the hotbar on top of it. I will freely admit to having a similar bar, because I am too lazy to keybind Healthstones or Mana Pots but having them pretty much connected to grid means easy access (manual clicking badness).
6. Drama vs General Vezax Hard Mode (Heroic) - notice the camera, max distance. More importantly the positioning so he can see incoming Shadow bombs and avoid. Basically repeating point number one but it is that important. With Quick Q/E keyboard strafing they don't have a single raider (mage elemental eats it though) get hit by splash damage. Simply by scrolling max distance and positioning cameras, a simple but powerful tool. Also look at where the focus target, player box etc are. Right by the persons feet, same reason why as a healer you want your Grid by your feet. Quick eye movements, the ability to absorb info while still keeping a focus on AOE. Players at this level take MINIMAL AOE damage, and I mean if they take more than 1 tick of AOE they will get sat for the night. There is no need to be that hardcore, but they tools they use to achieve that level can be employed by any level of player to improve his/her game. (kind of pulls all the previous points together! Dontcha think?)
7. DBM - /range will give you a box that will name anyone within 10yards range of you (range can be changed through settings). For Kel'thuzad, or any boss with chain lightning this is a blessing. Use it.
8. If you really suck at avoiding AOE and are tunnel vision prone - Bigwigs. The sides of your screen will glow and alarms will go off if you are standing in something, giving you time to run out of that aoe before you get splattered.