I've centred on using an "Onion topology" for my campaign preparation. I also use a technique, both for campaign and adventure site preparation, whose name (not concept) I've borrowed from the crowd at Dream Pod 9 called "The Y-Cubed Law".
One mistake I used to make all the time was lavishing waaaaaaaaaaaaaaaaay too much detail on things the players were never likely to find out and, thereby, rob my players of attention to details they were going to be facing constantly in the course of ordinary play from day one. (I blame I.C.E.'s old Campaign Law supplement for this since its step-by-step instructions started with the geography of a whole world including plate tectonics.) The cure, I later found, through much bitter experience, was to stop doing top-down design of my campaign and instead work on it from the bottom up. I later termed this "onion topology".
In onion topology you imagine your campaign world's information is arranged in layers like an onion. First you sketch out a whole onion in very rough detail – this would be things like gods, major continents/nations/whatever-suits-the-scope and other extremely broad-brushed details like that. You then move down to the core of the onion which is where the players are. That core is the immediate environs of where the players will start. In a typical pseudo-European feudal setting (which I will continue using as the example) it could be a single fief with attendant keep, village, outlying manor houses, etc. This you give in detail. You make plans for the keep and the manor houses. You make a map of the village. You place it somewhere in your "whole onion view" and draw up the local terrain according to the rough geography you have already put in place. You populate it with key NPCs (right down to the stats if this is important to your game). You find opportunities for plots via these NPCs. (Reading Georg Polti's The Thirty-Six Dramatic Situations is very helpful here!) You situate any typical adventure sites you want to use in the immediate future: haunted keeps, caves of despair, etc. Then, when everything is ready for your players ...
... you continue planning. Because players never do what you expect them to do. So you fill out the next layer of the onion to nearly the same levels of detail. Maybe you drop some of the finer details like minor NPCs, but you really need to detail one layer of information removed like the overlord's own fief, surrounding subinfeudated neighbours, etc. – anything, in brief, that the players might go to visit on a tangent.
Now you're ready to play. You have enough of a broad-picture view that your campaign isn't completely ad-hoc. You have many details for a smallish area in which you can situate your immediate-term needs for adventures and you have details for a larger area into which your players may suddenly sidetrack so you're not caught with your pants down. From here on in your campaign planning work is easy. Run your adventures. While you're doing that add little details to the next layer out until either your players branch out into it themselves or you're ready to branch out. As soon as they do, you start work on the next layer of the onion, always keeping your campaign details planning one layer removed from where the players currently wander around. This way you have the benefits of planning a campaign (especially the consistency) without the burn-out and the wasted effort. When your campaign finishes your detailed campaign notes will be a patchwork quilt of details and sketchy stuff, but your players will never know. To their eyes you'll have had this wonderfully huge and gloriously detailed campaign that they never reached the boundaries of.
The Y-Cubed Law
Hand in hand with the onion topology I used, since the early '80s, a technique I later grew to call "The Y-Cubed Law" courtesy of the boys at Dream Pod 9. Where the onion topology gives players the illusion of campaign breadth, the Y-cubed law gives the players the illusion of campaign depth.
What I found tending to happen both in my campaigns and others' was that in a bid to bring a place to life people would put in all sorts of interesting local quirks. For example: "In the village of Hothentot the temple bells ring once every ten days at 37 minutes past noon." Or: "In the land-locked city of Kamesh, right in the central square, there is an enormous, 1:1 scale statue of a whale." These are harmless little details of no meaning or merit; just flavour text, really. But ... players will always arrow in on these details and start asking questions. "Why does a land-locked country have a statue of a whale? How would they even know what a whale looks like?" ... and so on ad nauseum. As a player I always found the GMs floundering and looking, frankly, a little stupid as they get blind-sided by such questions and as a GM I floundered and felt, frankly, a little stupid when I got blind-sided.
Enter the Y-cubed law.
In the Y-cubed law, any time you introduce a detail into your setting (or, at its most extreme, into NPCs) you ask yourself the question "Why?" three successive times like this:
- Q: Why does Hothentot ring its bells once every ten days at 12:37? A: They are celebrating the defeat of a horde of bandits.
- Q: Why ten days? A: The bandits were defeated by ten doughty adventurers.
- Q: Why 12:37? A: The time represents the ratio of defenders (ten adventurers plus two hardy locals) and the number of bandits they killed.
There. Now a piece of local colour has something that will stand up to a bit of casual curiosity. It also happens to provide some ideas for possible future adventures. (Where are the adventurers now that the villagers have the wherewithal to actually properly pay them? Were there any surviving bandits who'll come back for revenge?) The total time to ask and answer the Y-cubed can be, after some practice, thirty seconds or so for each detail and yet that minimal time and effort not only gives the illusion of depth but also gives the GM plot seeds that he can use in the future, thus saving time overall.