It would help to clarify if you're talking about 10man or 25man. But anyways:
10man: This is rather easy in my opinion so I'll just post the obvious things if you're talking about 10man. Make sure that everyone stacks up in the middle area shortly before the Valkyr is about to spawn. It's important though that people move out ASAP when the Valkyr has her target since the Defile is often coming pretty closely after the Valkyr. Have people with stuns call them out before they actually do them on Vent or any other voice chat you're using so that they won't waste a lot of those with diminishing returns. Keep a hunter trap / earthbind totem on the boss so that it gets slowed instantly and make sure people know the worth of their slows compared to just DPS. I don't know what else to say about 10man since there's only one spawning and it dies so fast.
25man: Make sure that everyone stacks up in the middle area shortly before the Valkyr is about to spawn. It's important though that people move out ASAP when the Valkyr has her target since the Defile is often coming pretty closely after the Valkyr. <- That one counts for 25man, too with one little addition. Make sure everyone is standing on one side of the Lich King (preferably the back) so that all Valkyrs WILL fly into the same direction. This is the biggest DPS boost you can get. Then, have 3 teams dedicated to each Valkyr with their own slows and stuns. Mark them fast (preferably an addon doing that for you) and have people DPS their assigned Valkyrs while slowing and stunning them and then helping out with the others if theirs die first. The helping out part is incredibly easy if Valkyrs fly into the same direction.
I don't know what else to say but if you got any other questions, go ahead. Good luck.