In addition to the stuff in Yeol's General tanking tips link above, I really suggest getting used to the enmity bars in the party list:



Those small bars that sit on top of everyone's job icons in the party list show the aggro status of your current target. If your own bar is red, you've got aggro. If someone else's bar turns yellow and gets close to filling, it means that person is close to ripping aggro from you and you might need to do something about it. To my knowledge, these bars aren't explained ingame at all but they're incredibly useful in optimizing your tanking play.