While I think a threat meter like Omen is required or even paying attention to the Blizz UI threat warnings, just following a few simple rules like those mentioned in the posts will prevent most problems.

1. MARK the mobs. I know the bright players use target of target, but put a skull on the one you are going to kill first please - it really helps the DPS.

2. CC. Check how many party members can CC and then make sure you MARK them and get the appropriate member to root, sheep, hex, chain etc. Oh and try not to CC the casters, there's not much point ;-)

3. Kill order. You have all those mobs marked - make sure you follow a consistent kill order or if you need to change it - TELL everyone when you are.

4. DPS WAIT - then WAIT some more. Most tanks need a few seconds for threat, so let them get it.

5. WATCH the threat meter or the Blizz UI. If you are getting close to stealing threat - STOP.

6. Heal thyself and don't stand in fire. Mana is a lot more important to healers now and they don't have it to waste on your mistakes. So use whatever mitigation you have, avoid damage, pop any healing or mitigation cooldowns when needed (remember flasks?) and click on the goddamn lightwell if it's there.

7. You're still alive, the mobs are dead. WAIT for the healer/DPS to regain mana.