I found this out on my private server when I was just goofing around. You jump off of something when the elevator is coming down to you and it moves fater then you. When the elevator reaches you you will float in the air where the elevator reached you. If there is anywhere where an elevator has a ledge partway up where it stops, then this would probably work on a live server.
Here is a video
[ame]http://www.youtube.com/watch?v=jzICqSSmxeU[/ame]