I am, frankly, a terrible planner. I procrastinate and get easily distracted; I cannot be trusted to keep to a schedule most of the time. I'm also not very good at predicting what my players are likely to do in an upcoming session, unless they tell me in advance and, even then, there's no guarantee they'll stick to their plan. So, even if I had the time, interest, or ability to produce reams of material in advance of play, experience has taught me that my players will always go somewhere or do something that I hadn't anticipated and I'm once again thrown back on my own quick-thinking and improvisational skills -- which are skills anyone who wants to referee needs to possess regardless of the RPG they play.
If I have any virtues as a referee, they're these:
- I Read (and Have Read) A Lot: I've always been a voracious reader and that's given me a very wide pool of ideas to use as inspiration.
- I Have a Good Memory: I remember details very easily, which is good, because it means I can call to mind both what I've read and what's happened before in the campaign, the latter of which is particularly helpful in forging connections.
- I Have Little Shame: As good as my memory is and as quick-witted as I can be, there are still plenty of times when I am at a loss for how to proceed in the game. Luckily, my players are an unending font of good ideas and I have almost no compunction about swiping those ideas and incorporating them into the campaign.
The "collaborative" process in my campaign goes like this: 1) I am lazy and leave large areas of the game world undescribed, 2) My players seize on the lack of description and add some details as needed, and 3) I later embellish those details with further details of my own. Rinse and repeat. Now, if someone wants to praise me for having hit upon something uniquely wonderful in the annals of the hobby, I won't stop them, but, from my perspective, I'm just doing what I've done for many years now: turn lethargy into a virtue. When I was a younger man, I had both the time and interest to devote to churning out pages upon pages of maps and setting information -- one day I ought to share some of the stuff I created back in high school. Nowadays, I'm both too busy and, frankly, too disinterested in building a top-down setting to even attempt to do so. Plus, I've come increasingly around to the notion that top-down is not the way to create a fun and flexible setting for the kinds of adventures I like to run.
Much of the "collaboration" that occurs at my game table is small potatoes stuff. For example, if I don't immediately come up with a name for a random NPC, one of my players might make a suggestion of a name. The same goes even for personality traits and background details, but, in every case, I retain the final say. When the players where in Yethlyreom and sought out the leader of the cult of the Iron God, Phaedra, it was suggested that she might be a reformed necromancer and, cool though that idea was, it didn't fit with the vague idea I had in mind of who Phaedra was and why she led the cult, so I rejected it. Now, I like the notion of a reformed necromancer, so I've stored it away in my memory and will probably use it later on in a different context. That piece of player input isn't lost, but neither was it accepted uncritically. Instead, it'll stew a while and (maybe) resurface later, perhaps combined with other notions I shamelessly swipe from my players.
This is all I do. As I said, I don't see it as uniquely praiseworthy; I see it as the kind of stuff all referees learn to do if they're running an open-ended campaign in any RPG and aren't singleminded obsessives who know the names, stats, and personalities of every NPC within a 1000-mile radius of the PCs' current location. That said, I dearly love the way Judges Guild used to describe hexes within the Wilderlands setting -- a sentence or two at most to get the referee's creative juices flowing and make the setting his own. If I weren't so lazy, I'd probably do something similar with Dwimmermount, too, but I usually am, which is why I take the easy route of improvising and riffing off my own players' ideas.