Out of the three games I'm running at GaryCon, only one is self-written. Writing a one-shot adventure isn't really that hard. It's just a matter of keeping a few things in mind.
Have a clear goal presented
Sandbox games work great for your home game. But for a one-shot game, having a plot of "You good. Monster bad. Save princess." is okay. Just make sure that you spell it out for the players as soon as possible so things get on track. At a con, you usually don't have to worry about people trying to derail your game.
Lose the random encounters
Again, another home game device that adds little to the convention game. Random encounters just slow the game down when you're doing a one-shot. Don't put an encounter in that isn't directly applicable to the plot, and your players won't feel that they're wasting their time.
Don't rely on any special abilities or skills to resolve things
Players can be funny in what they choose. As a result it's hard to predict whether or not they'll choose an ability or skill needed to complete the task you set out for them. If the players roll up their own characters, they may be out of luck come the finale. If you're using pregens, then you risk making the final encounter dependent on one individual. Keep your resolution dependent on the actions of the group and not an individual. That way you make it easier for everyone to have fun.