I know there are other posts that detail how you can stay flying in Dalaran and actually move around, but since this particular bug hasn't been posted yet, I thought I'd add it in.
Most of the time, when you mount up, fly over an area you want to stay on your mount on, and then fly straight down (press X) without any horizontal movement, you will not get the dismounting debuff and wont be kicked off of your mount once you touch the ground.
Here's a step-by-step
1. Mount up at Krasus Landing or The Purple Parlor
2. Fly over the area you want to show off your flying mount on. Make sure you're flying high enough at this point so that the dismount debuff isnt on you.
3. Fly straight down (press X). Do not move horizontally at all. DONT STOP until you hit the ground.
4. Enjoy! Dont move horizontally or take off, and you wont get the dismounting debuff.
This works 8/10 times. Give it a try, and you'll see that it works. I tested this using both the celestial steed and bronze drake, so I know it works with mounts that can scale to ground speed and others that cant.
I know this exploit may not be as useful as the one that can let you fly around without dismounting, but hey, it something. Someone might even find a better use for it in a place like WG or Tol Barad later on.
Enjoy!