You saw a bit of what I did for campaign prep with the Order of Thoth in this post. I created similar documents for the Elessarians, Viridians, and Elves. The campaign will be starting in the eastern half of Viridistan in the Province of Gormmah so I need to get some details straight about the Viridians. Two of the players are Elves so I wrote up that document. Finally the Rusty Battle Axe wants to know about Elessarians so I wrote up that one as well.
Now I didn't really write 10,000 or so words in the past two weeks. The benefit of having 30 years worth of campaign notes is that I have a lot of stuff to draw from. Much of creating these handouts involved cutting, pasting and editing. Right now the above are serviceable as first drafts and campaign handouts. They will need further work before incorporating them into projects meant to be published.
Some parts are overlong, I am still working out aspects of all three races/cultures. For example with the Elves an important part of their history were the Voyages of Sorrow. The fact that the voyages occurred , and that they were a result of the Crystal Wars were a part of the mythology of the Majestic Wilderlands since the mid 80s. But I never figured out exactly why any of them took place. So I worked it out when I rewrote the current revision of the elves. Now I have several interesting variations of the base elven culture.
With the Viridians, I lost 3,000 years of their history including the darn king's list. Just disappeared 10 years ago and all I have left are the earliest years. The annoying part wasn't that I lost all that stuff, but it was the only place where I recorded details about the First, Second, and Third Empire of Viridistan. How they were formed and how they fell. So I re-worked out those details with the new revision of the Viridian race.
I am sure by now some of you are wondering what kind of crazy loon I am for going into this level of detail. Generally how my best stuff is created is that it originates in a king's list or a time-line that for 99% of you would be pretty boring stuff. This includes my players. So I give them the reader digest condensed version of it. In the earliest days I didn't do a good job of condensing, but I learned. I kept at it over the years until the documents were useful and contained the best and more interesting ideas I had about the subject.
And sometime those old notes become handy when some obscure detail generate something really cool. Now you don't have to do this for your campaigns, but I do recommend that you keep a folder or notebook filled with idle thoughts, useless details, and other minutiae that you happened to come up with. It may be that two, three years down the line you will be able to create a really fun adventure out of it.