8
down vote
I run one-shot Shadowrun missions at conventions. A couple of things I can offer:
Use pre-generated characters. If certain characters are critical for mission success, make sure they get played, or NPC'd. Some missions are more flexible than others.
Keep the mission simple. It can have a big plot twist, or a dark, oppressive tone, but it shouldn't involve a lengthy, multi-stage investigation or setup. Figure out ahead of time how the players will (most likely) try to beat the adventure, using the clues you give them. Use the "Five-room dungeon" model as a guideline.
Impose an in-game time limit on the characters, and put pressure on them periodically to get the job done. This is a big one. Remind them every so often that time is moving forward, as they go about planning and doing legwork.
Keep the game moving. Players will spend the first three hours of a four hour session preparing for the meet with Mr. Johnson, if you let them. Railroad them over the boring parts, and say, "You're there." instead of asking where they want to be. Let them have more freedom when they get to the interesting parts.
Make sure everybody at the table gets a turn, not just the loud ones. Shifting the spotlight from player to player also helps to keep the game moving.
Watch your time. Gloss over things, and make the run a little easier, if you're running behind.
Make sure combat runs smoothly. I find that using index cards for initiative is a big help. When there's combat involving only some of the party members, keep them short, and try to give non-combatants the spotlight, too, between rounds.
If you need to save time, gloss over complicated procedures (particularly hacking) with a single skill roll, or teamwork test.