3. Manage enmity

Don't just keep attacking when your enmity light is about to go red (next to hostile creature list). Either stop attacking all together, or cycle through targets that are green to let the tank catch up. Resume attacking the primary target, when the light goes yellow, or if it is about to die in few moves.

If your tank is a Paladin, other mobs will usually be cc'ed, and most of the time, they don't have enough enmity built up in other targets, so just stop. If your tank is a Warrior, you should be able to cycle through targets no problem.