My Results.
OP here for those of you who don't remember. I've been keeping up with this advice... why else ask for it? And I thought I had might as well show what I've come up with. So here is my new way of Prepping to Improvise For Overplanners.
I've been using OneNote since the second session, and I'm happy with it. But there were a lot of improvements I could make to the way I physically write up my sessions, and how I organize the information. I've reorganized all my information more intuitively, and practically copied the entire SRD into OneNote, so I never have to leave the program, navigate to the book, index the page I want, read, close, and then remember what the hell I was doing in OneNote. It's also entirely sized to fit in onenote on a half-screen, with everything visible, so I can keep MapTool visible (It's an online campaign with RL friends... that's what happens when your DM moves to Finland).
Screenshot:
http://i.imgur.com/hbvkM.jpg
Plotlines and Intrigue are going to make my life a lot easier. Previously I was writing adventures like a novelist. I'd sit down each week and write the next chapter. This is (for me) a time sink! Instead I've got all the plotlines (two majors, several minor, and one for each character arc) on seperate pages, with a brief summary and a list of bullet points briefly documenting what's happened so far. All I have to do is add a new bullet point to a couple of them and I've got the foundation for an entire adventure. In the Intrigue tab, I've got all my factions listed, and similarly, what they're currently doing. Now I can just KNOW that for when I have to improv any member of any faction, and its completely reactive to the story. The Heroes tab includes character sheets and backgrounds, Improv is all quick lists of npcs or events I can throw in on the fly. DM's lair contains all of the collected wisdom I've found in my quest to be a better DM.
The big directories at the end are fairly obvious... a campaign enyclopedia, rules index, past session notes, a place with a lot of statblocks (can be used for improv), and an index of NPCS... I need two, because half of the campaign is urban and it's easier to find the right npc quickly when in the city.
Finally I've thrown together a series of templates. The General Encounter one is showing in the screenie and encompasses pretty much everything needed to run anything except a skill challenge, and I can then attach subpages to it with stat blocks as needed.
By adhering to these, I feel like I've got a complete and viable session plan, but I've stripped out all the extraneous text that happens when I overwrite. Vital new additions to my process are knowing what exactly the point of this encounter is to the game as a whole, and what might happen if the PCs fail. That's going to make me a better DM, because I'll be less shy about allowing them TO fail. I'll know how the story moves on.
The other templates are visible there and most include reminders and other things to have prepared, as well at things not to forget when the session ends. The Optional Encounters contains very brief ideas for on-the-fly stuff I can drop in if the pacing goes downhill.
Aaand finally, the Overview templates are the control centers. For a Site-Based adventure, they've got background but possibly relevant information... what the place is, what it was originally intended for, goals the PCs are there to accomplish, and dynamics. By Dynamics, I mean it's a quick bullet point list of how the dungeon and inhabitants will react if certain things happen. In the Event-Based template, it's fairly simple; An encounter I want to run with assigned number, followed by bullet points about how I think the PCs can most likely get from one to another, and repeat.
Last big change is probably the way I do NPCs... I've stripped out well over half of the fields I used to fill in for each. I learned from a couple of articles I found this week that my tendency for complete descriptions is completely unnecessary (the ones about how Stephen King writes, by Chris Perkins on the Dungeon master Experience, are the best explanation of this). So, dropping seperate fields for hair, eyes, clothing, background, etc, has left me with the essentials. Race Class Level, Appearance, Voice, Traits, Goals, Motivation. And I'm thrilled because I now realize this is all I need, in 1/3 of the words I was doing it in.
So all in all, this probably looks like a lot of prep still, but I know what I was doing before. This is much more efficient, allowing me to have a plan I can reference extremely quickly, but the structure is such that I write only the skeleton of the adventure, not the story itself. It's just less convoluted than trying to write from beginning to end what happens. Thanks everybody for your advice and examples... I'll be trying this new way out tommorow and see if I can keep under 2.5 hours!