This is pretty hard to do- but I'll try to explain it as best I can.
1. Get on your flying mount.
2. Take off.
3. Press '/' on your numpad to walk.
4. Land, but keep moving.
The next few steps have to be done almost at the exact same time.
You must also hold down 'W' for steps 5-6
5. Right click the mount buff
IMMEDIATELY AFTERWARDS
6. Jump and continue to hold space and 'W.'
7. While holding 'W' press your auto-run button. Mine is configured to the side of my mouse.
8. Release 'W.'
9. Release Space-bar
10. Ta-dah!!! You're done.
Please note: It is ridiculously slow going if you tried to actually get somewhere with this, so show it off in Shattrath.
WARNING: NEVER PRESS 'W,' 'A,' 'S,' or 'D' while performing this exploit. This will just make you start running again- you MUST use the right mouse button.
Edit: Sadly, this is client-side only. Oh well. Have fun with it. First thing I really found out myself.