I find that I prepare differently across the course of a game. Sometimes I write out character statistics, available items in shops (with NPC information and pictures), detailed adventure synopsis, clues, and sort out scene by scene playlists. Other times I have a vague idea and some dice. Sometimes I have an idea of what's likely to happen and a few pregenerated enemy sheets. Sometimes I haven't a clue and just hope something comes to me.
Since I run multiple games at any on time this preparation can also vary every few weeks. At the moment, I prepare a lot for Dystopic with clue cards, lists of leads, NPC stats and basic descriptions, individual sountracks for the likely locations and scenes, research into mechanics, and print outs of all the player character's merits and lores (because they're spread over soooo many books). In the first few sessions I winged it with a bare idea.
When I first began Flashpoint, I sorted out a whole bunch of information on Andoran, made a map (really must post that at some point) of Augustana, did up some shopping sheets, found possible factions they should join, and designed several NPCs and enemies. I also did a lot of research on Westcrown, read up on weather effects, printed out paper miniatures, and worked out how five very disparate characters might wind up together in a very interesting and distinctive way. No mere tavern connection for them. Now I've got a vague idea and a few Bestiaries full of Devils to work with. It's not completely winged, though, as I have a fair idea where I will send them.
In truth, since there's no real trap smith amongst the Pathfinder PCs, Dystopic will likely need more prep work than Flashpoint for an average adventure but that doesn't mean Dystopic will always get a high level of prep. Sometimes I'll just rely on my intuition and improvisations. Sometimes I'll focus on the solo campaigns to the exclusion of all else. Sometimes I'll tell them to go read a book and I take a week off.
How about you guys? Do you do a certain amount of prep work per session? Does it vary considerably or not a lot?