The most useful advice I can give for running convention games is to constantly ask the players questions and riff off their answers. A lot of players new to the system will be unfamiliar with having that kind of direct input into the story and will think it's cool that their ideas drive the game in an immediate fashion.
I think the hardest part of running a convention game is in pacing it properly. You'll want to build to some important denouement, something that gives the players a sense of accomplishment, and you'll want to time it such that that happens at the end of the session. This can be tricky depending on what's happening in your particular scenario.
And speaking of scenarios, I don't plan one. At all. I go in with a totally blank slate and let the plot be built from the players themselves. I've found one of the best things to do is to start them out in medias res with some situation, and ask them to fill in the details. Like, "OK, you guys are all in a pitched firefight. Keeler, who are you fighting and why?" Once you have their answer, "Sweet. Burroughs, where is this firefight taking place? What's around? And what are you currently taking cover behind?" Build off that and move to "OK, Damson, how is the fight going? Where are you and what is your immediate objective?" Flesh out the situation off their answers. And don't forget the Hx round, because that will give you scads of useful detail too. Using this approach I once started a convention game (with three players) where the first scene had two of the PCs tasked with effectively kidnapping the third, and it was hilarious.
One additional thing I've found that's fun is to do an epilogue, where you ask each player: "now that X has happened and Y has been revealed, what would your character want to do next?" This focuses players on the idea that this session could lead into a longer story, and drives right to the heart of the old adage "always leave them hungry for more."