Hey guys! As a prot warrior i know that is hard to keep the aggro from all the mobs that you've pulled while dpsers do their job...This is not much of a guide but it sure help in starting to understand the consept of multiple target tanking.
First of all in ALL instances and raids its very important that you let the tank pull.
No ninja pulls = a safe raid = no wipes = less money in repairs
As long as you have pulled the mobs you'll need to tank em all as fast as you can so the dpsers can start earlier and healers have no trouble with aggro trying to heal you and the raid.So you need you start with this combo that i think its the best way to keep your aggro up.
Bloodrage(just before you pull to get your rage up) /demo shout /TC(thunderclap)whenever possible / devastate x2 on a target / tab / shield slam / devastate x2/ tab / and it goes on.
try to keep 2-3 sudens on every mob if possible. shield slaming a target is the most high threat that you can cause in one shot.
Demo shout and TC are very usefull aoe threats in a tank warrior and its important that you keep em up when multiple tanking.
Ofc dpsers and you as a tank should have a killing order.dpsing or tanking a mob that isnt a priority will cause you losing the aggro on that mob.
In the case that a mob has escaped from you taunt/shield slam/devastate x2 will make it come back and keep it there for a while. in immune to taunt mobs stance dancing for Mocking blow is very helpfull ( "/use Battle Stance (enter) /use Defencive Stance" is the macro for a tank that will help you with stance dancing )
I hope i have helped a little.It takes some time to get used to it and always is a pain in the ass for tanks to multiple tank but it is a must in tanking.
Thank you for reading enjoy