One shots (or One Night Stands as they are known to the more vulgar part of the roleplaying community) are exactly what their name suggests - one time scenarios, that are run without any intent to developing into a campaign.
There are a few good reasons to play One Shots. First of all, they can provide some fun and interest between adventures in your regular campaign. They could be a good chance to try out new genres and settings, or just to get wild like you'd never dare do in an ongoing campaign. And above all - if you're playing at a convention, you're playing a one shot. As simple as that. This article's purpose is to provide a few rules of thumb on how to design a good one shot scenario.
You're working under a time frame. Usually, you won't have the opportunity to continue the scenario in a later situation the way you can do with your regular campaign and players. Find out in advance how much time you are going to have, and plan a scenario that will take a bit less, so you have some spare time to toy with. Determine in advance how long each scene should take, and try to stick to it. Never cut a scene short just because it ran out of time ("The were unable to injure suddenly turns around and runs away"), but you can definitely discard less import details or even scenes in order to stick in your time frame.
When running a One Shot, you will often be playing with players who don't know you (especially true for scenarios run on conventions). You have to get their interest right from the start, or you will loose them altogether. You don't have time to describe an elaborate campaign setting or background. Explain only the utmost necessary details at the beginning, and let the players discover the rest on the way (this is an especially useful tip for scenarios of the "You Wake Up In..." type, which are quite common as one shots). A powerful dramatic (or gory, or frightening, or hilarious) opening scene can definitely contribute.
Use background music (some good tips on this subject can be found in the article The Sound of Music, here at the Hall of Scrolls), and physical props. Play in a dark room lit by dim candles. Dress according to the scenario's genre. No gimmick is too cheap to get the player's initial attention, or keeping it.
Have you ever created a character that was extremely interesting, but you felt it was too unplayable for your campaign? One Shots are an excellent opportunity to use such characters. Remember - One Shots are all about getting a different playing experience - create characters accordingly. Overpowerful monstrosities, pathetic wimps, characters that contain too many internal conflicts to be playable, characters that are natural enemies of other characters in the troupe, hard-core villains - everything goes.
Plots should also contribute to a different playing experience. One Shots are a perfect place to experiment with Character Driven Scenarios (i.e. - the plot is not the center of the scenario, but only a tool to generate situations that would be interesting to roleplay as the given characters), Idea Driven Scenarios (the plot is just a tool to express an idea, concept or philosophical debate) or Point Driven Scenarios (the plot is races towards a completely unexpected climax).
Remember that a One Shot is a one shot. It is not supposed to be a part of an ongoing campaign. End the scenario in a clean-cut fashion, tying up all the ends. Make sure the final scene answers all the questions (or at least most of them), instead of creating more. You can also benefit from using an epilog scene (preferably, with you only describing, and no active playing), to show the players the consequences of their actions throughout the scenario.