I normally do standalone adventures without much of an overarching plot. But my players are really looking forward to my running more of a real game with such an arc. Worldbuilding is easy, but organizing NPCs, factions, background events going off, chains of events... well, there's a reason I didn't do well in school.
So what tools do you have or tips that you use to make it easier for you?
I need to note that, until nearly age 50, I didn't have any memory issues. My ADHD resulted in having distraction and digression bomb issues. Wait, no, about 45, the PTSD started... which is a memory issue, but not one of forgetting... but remembering things at inappropriate times and from inappropriate triggers. (Writing this is triggering memories of intimacies 30 years ago... and of holding a student's hand as he died on the street, 20 years ago, having been {Spoiler for traumatic situation}[o]hit by a semi along my route home after work; yes, he was a student in one of the classes I was the teacher for. Kindie. Such a goddamned waste. I can even smell the diesel and gasoline exhausts, taste the road salt in the air...[/o]). And of being about 14 months old and looking up at the furniture in the other room. And of Great Granddaddy in that other room, years later, Christmas of 1976. And of helping catch the goose for Christmas dinner.
My players, all currently younger than I, also have digression bomb issues.
Organizing my notes was never an issue - but
reading them was.
Techniques I've used to keep on task, however... they do tend to overlap.
:d10-1: Run published adventures. The best have needed nothing else. (WFRP's The Enemy Within for 1E even has cheat sheets for the NPC stats already in some volumes!)
:d10-2: work from outlines. The Major Plot Arch needs to be schematic - the more detail, the harder it is to bring to fruition. This was very vital to running BTVS, and very useful for running a homebrew collection of Sentinel Comics. I also write many adventures as outlines, rather than full prose.
:d10-3: Notes taken during play.
My current sunday group, I've a single file, with every major NPC I intend to reuse having a one liner - Name, Race, Class, what they look like (usually an actor), a couple words of personality and a goal, and if a caster, what spells.
:d10-4: booklets of NPCs.
My Dragonbane notes for an adventure include, in order, the NPCs for each encounter, how many, and tracks for HP and WP points.
I really should be doing those right now, but instead, I'm doing this.
You can extend this to factions, too.
:d10-5: Writing Session Reports
I don't do the session write-ups I do on DTRPG for others benefit - I do them because it's useful for fixing the important bits in memory. That it's useful for others is a nice bonus.
:d10-6: adequate medication
For those with meds, not just the GM, but the players. If everyone's medicated properly, the game goes more smoothly.
:d10-7: Journaling
Keep a log of what happens in the background with major factions, and how the player actions have affected that. It doesn't need a lot, but just a "What they've done"...
I'll also note: I've done the journalling in TiddlyWiki before. I found it an ideal tool, due to the hotlinking and on-the-fly page creation - while keeping everything in a single file. YMMV.