I've noticed a pattern in the past few days where my monk's enlightment buff stays at 0 seconds left until I log out. I've finally been able to reproduce this and it's easier than any of the methods already available.
Step 1) Get enlightment
Step 2) Wait until it has ~10 seconds left
Step 3) Get into combat, and wait until buff is supposed to expire. It should stay at 0 seconds,
Step 4) Get out of combat. The buff should still be still be on and ticking at 0 seconds.
I've done this many times before and it works each time. Logging out and (in some cases) going through a loading screen will cancel it out.
I can confirm that you still recieve the extra XP.