If anyone in your party does manage to pull threat, always make sure to run to the tank to have it picked up.
Attempting to run away only increases the time it takes to get the mob pulled off, likely resulting in the character's death or even the whole party's. Monitor the threat from critical hits.
If you land a couple of high critical strikes in a row, disengage for a second then reengage. Critical strikes increase your threat, so consider this when monitoring your threat output.
If you have an off-tank in a party, don't use the off-tank's threat-generating abilities unless a difficult enemy breaks from the main tank or the main tank dies.
Pulling off of the main tank will interrupt his threat generation, and your healer may not have enough mana, or time, to heal both.
If the main tank loses threat, everyone in the party must disengage until the main tank has regained threat. Use any threat-reducing abilities, such as the rogue's Feign Death, if you have them.
Once the main tank picks the enemies back up, wait a few seconds for the tank to build threat, then reengage.