Okay, I know there are other ways to accomplish the same effect, but I dont think this has been posted before.
So it's very simple.
You mount up you (For example) Ashes of Al'ar.
Then (If your flying, make sure your on the ground) type /cast !Ashes of Al'ar
If you can't tell, thats an exclamation point.
tl;dr
1:Mount
2:/cast !Nameofyourmounthere
3:Your mount should be missing.
I tested this on a 3.0.8 ArcEmu server.
If you need me too I will post pictures.
Have fun!
Edit: Added pictues!