Don't be afraid to change targets when one is almost dead to help control the amount of threat you have on a mob. Generally when I am tanking a pack of 3 mobs or so, I change targets at ~30% and start working on the next one. Even if a dps pulls the threat from the one you are no longer attacking, its not going to be alive much longer.

Don't be afraid to weave some flashes in between combos. This can help with high dps. Might not always be necessary, but it is certainly better to get into the habit now, and later judge it later.

Like someone else mentioned, you almost never want the mob(s) facing the group. When I pull a pack. I run in, flash, CoS, then spin around 180 degress (going from 6 o clock position from pulling to 12 position) so that I know the mobs wont be facing the group. This also helps for mobs that patrol as they will run to hit you first.

You need to become a master at multi-tasking. You will need to watch your party window for your groups threat levels, you need to watch your target window for things the mob/boss is doing, and you need to watch your engaged enemies list to see your threat level.

Don't be afraid to run into a group of mobs, but don't rush the attack either. If someone dies and has to be rezzed that's mana that the healer has to recoup, that's time that needs to be spent rebuffing that person. Learn to judge these things.