How do you freeze your WoW Account?

To add to what Oppositeday and Lau said, Draenion, you simply need to cancel your subscription and let the time run out. We are not able to "freeze" the account in mid-billing cycle meaning that if you just paid your 30, 60 or 90 day subscription that time will still tick down.

Any auctions and mail you have will delete once expires so you'll want to clear those out before you depart but otherwise your character should remain how you left them.

My only word of caution is to make sure your system is secure before you depart and that your contact information is up to date.

*Important* Key-Loggers and Computer Security
http://forums.worldofwarcraft.com/thread.html?topicId=1778038509&sid=1

If you need to update your registered e-mail address contact our Billing Department.

You can call 1 (800) 592 5499 (1-800-59-BLIZZARD) Monday through Friday 8AM-8PM (PST) for live representation.
Australian users should please call 1-800-041-378 if the standard 800 line does not work.