This method is best done in a team with a lock and a healer. The best combination is with a priest or druid that have good healing over time skills, but any healer will do. This allows the team to kill enemies literally nonstop in a full run without having to skip a beat. The team will both level up extremely fast and get tons of good loot.
The lock will be doing most of the work here so make sure he knows what he's doing. The healer will need a lot of bag space as he will be looting all the kills. The healer should have plenty of water handy because he will be the lock's lifeline. Choose a spot with enemies at or slightly below your level that the lock is confident he can kill without too much trouble. It's best if you get a place that either gets a quest done or has good drops to loot. I recommend killing humanoids as they drop the best stuff. You want the enemies to all be close together and respawn quickly.
First, set it to master looter. The priest will be doing all the looting. Be sure to do this with a priest you can trust as you will have to split the spoils evenly after the run. When the lock is good and ready, he will run right past one of the enemies and drop on as many Damage Over Time attacks as he can. I usually do corruption, curse of agony, and siphon life. This will be much easier and safer if the lock has a run speed enchant on its boots.
Keep running right past the enemy and kite him behind you. He will fall into a nice line with you and just keep chasing you as you go along. You need to keep running and keep going in a long, wide arc so the enemies can't catch you. As you run by the next enemy, DOT him up and get him to fall into line behind you. Don't pull an enemy that's in front of you or he may actually catch you. If you get caught, you may get stunned and all the enemies behind you will all pile up on you and kill you almost instantly. Be sure to read my guide on kiting for more pointers on keeping from being caught. The key here is DO NOT STOP for anything, at least not until all the mobs behind you are dead.
Just keep running and DOTing more and more enemies without stopping. You will get more and more of them falling in line behind you. I used to kite up to 10 enemies at once. I had a crowd of enemies dropping dead one by one, leaving a trail of corpses in my wake. Meanwhile the priest follows behind and loots the corpses as they fall.
If an enemy doesn't die from your DOTs, just put more on him as you run. The goal is to be able to kill him with one set of DOTs, so if it's not doing the trick, you may want to go for easier enemies. Or you can just keep re-selecting them and DOTing them back up. When you get low on mana, life tap until it's back. Turn in a slow arc so the enemies don't catch you and run back past the priest. He will drop a heal on you and you're good to keep going as he scurries around cleaning up your mess.
If you get in trouble and you do get caught, pop out a howl of terror. This will send 5 of the enemies running away in fear, and allow you to run as they fall back in line behind you. Use this sparingly as it has a cooldown. If your healer is nearby, he can sometimes heal you through the beating you will get when your angry mob catches you. Don't rely on this because after a certain number of enemies attack at once, there's really not much anyone can do. This technique is somewhat reckless so be prepared to die a couple times if need be. Once you get it down, you will have no trouble laying waste to masses in no time at all.
This can actually be pulled off solo as well. This is actually how I grind on my lock. You just have to be a lot more careful because if you get caught by the mob, you will have little chance of survival. You also will probably need to stop every so often to mana back up. Make sure to run them around in a tight circle if you can so the corpses are scattered close by. This keeps you from having to waste a lot of time cleaning up the loot.