For a one-shot, here's what I like to do.
Simple, Clear Goals
Set up a simple goal. At no point should the players be stuck for ideas of what to do next - they might be debating between a few options, but never at a loss or scraping for clues. "Find the vampire, kill it." "Convince the king to join the alliance." etc.
As the GM, you should write down a single sentence or question that defines when the scenario is over - "Do the heroes escape?" "Who controls the throne?" etc.
A simple front and back sheet detailing the most basic rules so the players can reference it during play. It also serves as a guide when you're explaining the rules. It's short enough people can skim it during breaks or while someone else's turn is going on.
All of the PCs should have good reason to be involved.
All the PCs should already know each other. If it's a scenario in which they should be at odds, you should give them personal reasons to be at odds (conflicting goals, grudges, etc.). If it's a scenario in which they should be working together, give them all positive relationships to each other.
The scenario should be able to work even if the players don't play all of the PCs. Ideally, no individual PC should be required for this to work. Identify the minimum number of players you'll need, though.
Mechanically, make all of the characters solid builds. My general rule is make them 80% good- solid enough to hold their own, not totally tricked out or min-maxed. Not only does totally optimized characters make for harder balancing in play, they're also harder for players unfamiliar with a system to use. Also try to avoid the more complicated subsystems if you can.
I will usually write a few sentences of "How to play this character" with regards to mechanics - "So and so is a melee fighter, you want to get in close. Use her Thunderfist power when the enemy crowds up." Just enough so someone can have a basic idea of what this character plays like.
Pacing a one-shot is tricky. You have to ruthlessly cut scenes when they start to drag, even cut straight to the climax if need be. I typically run one-shots of 2-3 hours, though I've done 1 hour to 90 minute games as well. Mostly, you have to figure out how things are looking at the 50% mark of your timeframe and kick things into gear.
One thing is that if your game is hard to improvise for (like encounter-balancing, etc), just tell the players, "Hey, we're going to play straight linearly, since this is a one shot" and then you can skip them trying to figure out what to do. If the game is easy to improvise for, just drive your scenes towards answering that conclusion sentence/question you wrote down at the beginning of this process.