This requires you to not have completed the quest The Battle for Andorhal - Quest - World of Warcraft
Just go to the NPC War-Mage Erallier - NPC - World of Warcraft who is below the Alliance flight master in Booty Bay, pick up the quest The Battle for Andorhal - Quest - World of Warcraft and talk to him. He'll create a portal to Andorhal (in WPL) which you can go into as many times as you like, providing you don't turn in the quest.
I searched for this, and I could have sworn I've seen it somewhere here, but was unable to find it, so just thought I would make a quick guide about it just in case someone doesn't know.