What I do to organize things is that only one thing has to be continually reworked and that is the actual plan. The personalities, motivations, and overall goals change much more slowly.Also, I agree that there is serious between-game work that needs to be done to move the world forward incorporating the consequences of the previous session. You have to be content with constantly ditching old plans and reinventing.
When it comes to locales, I try to flesh the location out enough that it functions as a place within the setting and thus other reasons could crop up that cause the PCs to visit. Not to get the plot back onto the rails, but it makes sense it is involved in the new thing that happening as a result of PC action or non-action.
I documented a campaign below and there was only two things when I had to come up with a lot of new things at once. That is at D where the party playing a group of fantasy mercenaries decided to buy off their contract and seek work elsewhere (Point I). And point L where the party bagged an enemy king and sold his ransom. The decided to "retire" to order to build a crossroad inn that occupied the rest of the campaign. I winged it on the basis of my general notes for the region from D to I, and then again from L to N as both involved the party traveling.
Once it became clear what the party was intending to do, I fleshed out the stuff that formed the adventures around I to L, and then from N to the end of the campaign with the Inn successfully built. The campaign ran for 32 sessions from July of 2012 to July of 2013.
Also no my notes are not that incredibly detailed. This was run on-line using Roll20 because a couple of friends lived in other states. Whenever I do that I just log the date into the program I used, The Keep by nBos. And maybe if I am lucky I will write some notes beyond a short to do list as a reminder about what to work up for next time.
And it helped that the campaign just shook out to be have three distinct phases that make easy for me to remember more details than usual. And it helped that I wrote a pair of blog post right after in 2013.
Since the Summer of 2010 I been using NBOS's The Keep to keep track of notes for the various campaigns I been involved with. At a minimum I...
Part 1 The campaign takes place in Nomar, a Ghinorian Realm with many elements similar to the mythos of King Arthur. It is ruled by P...