Two things come to mind.
First, plan some "throw away" encounters. These are encounters you don't need for the plot, but that you can use if you need to "kill" more time. Maybe the barbarian scored three crits with his greataxe and the combat you thought would take about a half hour was done in five minutes. Maybe the roleplay encounter you planned on needing 15 minutes for was over after 2. Whatever. These encounters help keep you on schedule if the PCs are moving faster than you anticipated.
Second, if it's a one shot you can do a few things you can't normally do. The obvious one is kill characters with impunity.
But for another example, I just ran a game at PaizoCon where 4 of the 6 PCs went insane (they had Will saves to resist), and begin working to kill each other - some overtly, others covertly. I had pre-written little tidbits about their "new" personality, and what their new plans were. I would never run a session like that in my campaign - as expected, it was a blood bath, with every PC eventually dying. (The last PC - who managed to hold onto his sanity the entire game - was rowing away from the haunted ship in a little rowboat when the powder room on the ship exploded - he knew it would explode, since he set the fuse. I asked for a Reflex save, and set a real low DC. I figured I'd give him this cinematic moment where he sees a big ass flaming chunk of ship flying at him and he rows as hard as possible and just manages to evade. Yeah, no. He rolled a 2. Instead, he ate the flaming bowsprit for his final meal.
)
Anyway, as a one-shot - and using pre-gens, too - your players won't have any attachment to the character. They didn't spend any effort to make it, they didn't put much effort into the personality and such, and so forth. That gives you a bit of free space to try some different things. I used possession and insanity, but there are a bunch of places you can go when the players have no attachment to the PCs.
Oh, and one final one actually. If you're making pre-gens, make sure you know what all the class abilities do, and that includes the spells you prepare for spellcasters. It sounds obvious, but it's a potential game breaker if you pause for 2 minutes to re-read the rules on a spell or class feature.