I keep a physical notebook with a link analysis table (sort of like hyperlinking but simultaneously fuzzier and indexical, it shows relationships between entities, locations, and events and just like real world intelligence tools, suggests/reveals potentials and possibilities for the game, basically gives a space for grounded speculation).  Evidently tailored products like World Anvil or general products like OneNote or GoogleNotes can do the same thing but I enjoy paper pencils and pens.  Whether prep involves consulting the notes or adding new notes is predicated upon what's occurred in game.  In this way I sort of let the game write itself, aside from some initial geographic parameters and power dynamics, I don't ordain a BBEG ahead of the game but let one rise to meet what the characters have accomplished (it might not be a BBEG, it could be a cataclysmic event, or consequential diplomatic moment, or otherwise a literal of figurative "invention."

To me the benefit of this is the lack of frustration I see in so many DMs with "developed" game worlds and an additional pleasure in witnessing my groups' players have a space to really contribute to creation of the world beyond character's actions.  I mentioned in another post a line from a Samuel Beckett novel where a fish with an odd daily routine is described by the narrator, the paragraph punctuated with something to the effect of "But does such a fish exist?  Why yes it does.  Now."  

My style comes from being steeped maybe too much into, among other things, the philosophy of "what makes a good story anyway?" including some exposure to a variety of arts' incorporation of improvisation, chance, or random disruption/breakage Into the work.  It's coupled also with a significant amount of experience doing much more high stakes operational planning where the need to make considerable space for the contingency of the unknown in said plan is one's safest bet. There's a real life thrill in not really knowing what's behind a door, and in game space I indulge myself the privilege of sharing the game version feeling of that with my players.

One of these days I really ought to make a map of my campaign setting.  I have only a vague idea how far any of the places I have names for are from each other.  I have three major cities, one town, and a few names for regions in the main nation.

I tend to file off the details and run stories based on stuff from books or movies I like.  I do just about zero prep work at all, and just kind of "wing it" when it comes to encounters.  Given that I'm only working with Tier 1, that's been plenty.

Q: How much prep do I do?

About a week's worth, not steady but I do add bits here and there. Most of the hooks I use have built in encounters that can be adapted on the fly. 

I like to go into the first few sessions with a handful of planned encounters and a sheet of possible random encounters. I have some minor valuables on a table to roll on for treasure. I have every major encounter and possible significant treasure written down. The if or when the party finds them is up to the party. There's been alot of times the party just doesn't want to follow your plan, so adaptability is usually the word of the day.

I think the most important lesson about preplanning a session is to keep everything loose and floaty. You never know when a 'touchy feely' Kender is going to accidentally drop an entire craneload of netted, boxed cargo on your big boss on day two, crushing him flat and breaking his neck...before they even knew the boss was the boss.

Also, have an evergreen one-shot that can be shoehorned into the campaign ready to go in case of emergency.

Players going off plan makes for alot fun. I don't encourage them doing it but when it happens I run with it.