In order to do this, you will need friends, to be at least 75, and not yet completed the Amphitheater of Anguish quests yet. While this may rule some people out, I urge you to keep reading.
It works using the simple mob tagging method, whether that's an exploit or not is up to user opinion, Blizzard's banned people for it earlier in Wrath, but it should be safe now.
Simply put, hit 75, head to Zul'Drak, and work your way up to ...
The Amphitheater of Anguish: Korrak the Bloodrager!
From here, accept the quest and summon Korrak. He is basically a tank and spank. Tag him, then let your friends (mentioned earlier, I used 2 level 80s) kill him. At 75, I got 31,000xp. From here, drop the quest then accept it again. Korrak is a mindless and easy fight, rewarding masses of experience per kill. I leveled here from 75 to 76 in just under two hours.
Enjoy!