I don't know if this has been posted before, but I just found this out today by accident.
I presume most of you know you can fly ontop of the well in dalaran. Well if you mount up and into the well before your time dismount timer reaches 0, then you will be mounted in the underbelly. You can now walk out on your mount and fly low around Dalaran indefinitely.
Make sure you don't fly too high or the dismount timer will reactivate.