Man has three ways to act wisely.
By thinking: This is the noblest.
By imitation: This is the easiest.
Through experience: This is the bitterest.
-Confucius


Tanking is not as much rocket science, as it sometimes looks like. You need to know about basic mechanics of each encounter, just like dd's and healers too. But you can forget about other stuff only dd's/healers need to perform, while watching out for your own stuff. (Tankbusters etc.)

As a War: keep back 50gauge to perform Inner Beast, until you know when Tankbusters will occure. (while Mt'ing)
Learn about your deff cd's. How long does each last? How much and which do you need to pop for which Situation?

A good advice: Talk to your co-tank/healers about mechanics that requires coordination. Which Tank will go left/right to break chains at Shinryu ex? When will you pop Holmgang so a WHM can use Benediktion? Always remember: Healers are your best friends, the other Tank is your brother.^^

Oh and...don't use Onslaught at OS2 when he knocks you back...you'll just wipe the whole party...n-not that that ever happened to me! *cough*