For a one shot, I usually find a fun module that I like but would never get a chance to run it normally. For you I'd suggest figureing out what you like genre wise and go from there.
Once the idea is found, then the details need to filled in. Names of NPCs, plot devvices, its kind of like writing a story at this point.
In prepping for todays game, I reread my adfventure, got some ideas for neat encounters and some fun things for the PCs to do that doesn't really have to do with the adventure, and just help get myself in the mind frame of running.