I have a quirk. I know there will be a running on a given day, and I know the location where the party will be either investigating or at which they'll arrive. There will be an important building, or underground, or description of the area that I ought to have on hand. But as the game day approaches, I'll do nothing to prepare myself. I won't design the building, or sketch out the halls, or even look up the information I ought to have. Instead, I'll spend the whole week working on stuff that won't have the slightest influence on the party's game.
They may be in Hungary, but I'll be mapping parts of India, or Scandinavia. They'll be on the edge of fighting a horde of mummies, but I'll be working on large mammals dwelling in Africa. They'll want to spend the session exploring a half-cleaned out dungeon they haven't entered in a year's game time (which they're doing right now), but I'll be working out the trade products for Spain.
Why? Because I don't 'prepare' in the normal sense. I haven't in years. I have gotten so comfortable playing by the seat of my pants that I simply never bother to draw a single micro-map outside of a session. In the little wedges of time between the party cleaning up after the last room ("who needs healing") and the party deciding what to do about the next one, I'll sketch out the shape of the room, and think quickly to myself, there's a secret switch under that cornice there ... there's a spring that releases gas right in front of the fountain ... the fountain water produces hallucinations ... that's good enough.
I'll figure out what kind of gas out of the DMG if I don't think of an effect myself before the trap is sprung; I'll let myself wax poetic about what kind of hallucinations; and what's behind the secret door that the switch opens I'll figure out if they find the switch. I am so used to thinking on my feet, and pulling the various random elements of things together at the end so they all make sense, it doesn't seem practical to spend time outside of the session doing that.
I think the real key is having a sense for structure. If the dungeon is filled with large insects, even the places behind secret doors, then it should be considered abandoned; who abandoned it, and why? Obviously, because something really awful is at the bottom. Is it sentient? Probably. But if it's really deep down, it hasn't seen anyone intelligent in years. It might be lonely. On the other hand, it might be xenophobic. But lonely sounds like more fun. If the party gets down that far, and finds something really horrific that wants to chat for comfort, isn't that unexpected enough to make the session fun? And it might know something old and ancient. Or it might be just the gatekeeper for something deeper, something more horrific, even a passage to another plane. Or not. Maybe after fighting through dozens of giant beetles, worms, slugs, crawlers and jellies, through level after level, the last room is ... an abandoned - and emptied - treasure vault. Hm, that would be a pisser. Which is it, let me see. I should roll a die. But not yet. They're not down that far. I'll give it a couple more runnings. Maybe I'll think of a third option. Then I'll roll a die.
I know I really ought to map it all out and know weeks and weeks ahead of time, but somehow that never matters to me. The party can't tell the difference, and if I'm playing by the seat of my pants I'm never reading off anything. It's all in my head as I go along, so there's always eye contact as I describe whatever's coming out of my fertile brain.
Sometimes I get stuck. Sometimes I have a bad night. Sometimes, frankly, my mind is just blank. I've had a bad week, I'm run down, I don't want to control the mob (sometimes I'm up to eight players running); and on those occasions they are either patient with me or we talk it over and quit for the night.
I'm not sure a lot of micro-preparation would be useful for times like that. If my brain isn't ticking over, it's because in reality I don't want the responsibility of running that night. Not having to improvise probably wouldn't improve my mood.
It should be evident from the Wiki that in vast part the work I do on my world is macro-design. The internet has certainly made this easier; it takes two minutes with wikipedia to scan through the details of a town or village, to familiarize myself with the local setting and to start describing what the nobles are doing, what's the latest news from the hinterland or how well the city is faring after the last major event. Improvisation with this stuff keeps the material fresh, as the rhetoric changes from month to month. In the big picture I see eventual wars, possible natural disasters or the deaths of important peoples affecting the party ... when I need a shift in the campaign, I'll instigate it.
This pattern of running lets me work freely on whatever large scale trade/mapping/biological design feature that interests me on a particular day, without worrying what my party is doing. Then, when the party does wish to go to Egypt or China or Morocco, the maps are there ahead of time and the purchase costs at the local market is ready (and can be generated in less than two minutes). The socio-political landscape I keep in my own head, fed by constant reading of history and other non-fiction texts.
See, it isn't that I design my world ahead of time, it is that I know it. I know it because I live in it - and so do all of you.