One shots are hard. Time is always a factor. You never know how many tangents will come up. Plus you have to linearize the adventure to an extent the players understand quickly what they need to do, and don't derail themselves for too long making plans.
My suggestions would be:
Start with a strong transparent hook.
Something that would draw all or part of the players in, and would motivate them to temporarily leave the main story line to pursue the new story side line, because they know it will be quick and they can see what they need to do.
Only have a few scenes.
Keep it simple you only have a few hours. Come up with a few scenes and leave it at that you are working with a small temporal window.
Try to limit it to 1 or 2 combats.
I don't know why but combat tends to slow most groups down. Strategy talk, past glory tangents, map drawing, figure moving. I don't understand how that could possible occur, but it does. (Yes I understand why it happens.) Use it to your advantage.
Quick off the top of my head examples.
While stopping for gas and snacks going to (insert storyline proper location). The group gets caught up in a gas station robbery, a local gang has swung in for beer and smokes, they are armed and in the mood for a fight. I can envision a firefight scene with things exploding off the shelves, coolers getting shattered, chips and soda spraying everywhere. Perhaps one of the gang members is talking crazy yelling about what he sees. They flee, party pursues. Final showdown at gang house, where the party discovers the crazy gang member was high on third eye, and you have a future hook to bring down a supplier. After something like that your players will remember the one shot every time they walk into a Quick Stop gas station.
Or how about . . .
Gun shots in the neighborhood lead them to a woman who just shot her husband and is running from the scene. When they ask her what is going on, she is all emotional crying, he just wouldn't stop, I had to. You get the weapon away from her, and calm her down. Then her husband shows up. Two gun shot wounds and all it did was piss him off. Insert supernatural monster here, and reason.
What about . . .
Its memorial day, and all the people that have been visiting graves are acting funny. Players discover this when one of them visits a family grave. A local necromancer has set up ritual traps to allow the dearly departed to temporarily possess a family member that brings gravestone decor. All kinds of confusion, and man out of time jokes available.
Hope this helps,
BaH