The easiest way is to just let them die a couple times.
But in response to your threat problem areas, the easiest way to pick up initial threat after a trap pull, for example, is to hit Commanding/Battle shout immediately after the trap lands and the rest of the mobs aggro the group. Commanding/Battle shout generate a very small amount of AoE threat, but unless people open up on the mobs before they reach you, it's plenty to get them to focus on you as the tank. If you then heroic throw any caster, and TC/Shockwave when the mobs arrive, you should have no trouble holding aggro off of silly dps. If the DPS simply won't stop attacking before the mobs reach you, let them die. It's their fault and they should pay the repair bill.
Kathy, I said, "I'm lost" though I knew she was sleeping
I'm empty and aching and I don't know why
Counting the cars on the New Jersey Turnpike
They've all gone to look for America