Practice your rotations outside of a fight until you can do them without having to think. For a fight, focus on mechanics before DPS. Often times, you'll fail to simply not having enough people alive over an actual DPS check. Once you know a fight, that's when you try and push your DPS to your heart's content. As stated above, focus target is a godsend and will help pull your eyes back to what the boss is about to do.

Forget everything you learned in FFXI. I've had so many FC mates compare this game with FFXI and try to play it like FFXI; don't. It's like comparing a first-person shooter to a side-scrolling or top-down shooter.