My pendulum is swinging further away from improvisation towards writing more stuff down in advance. There are some good benefits to having written notes! You can feed the players more relevant information through rumors, patrons, divination spells, and so forth. You can drop content with a variety of danger into the setting, and generate verisimilitude by leaking information. The game world depth becomes abundant and unavoidable.
We spend a lot of time out here on the blogs generating our random tables. They're inspirational. They let us throw some dice and abdicate a bit of decision making, opening up different avenues of creativity. You know, the whole 'better to riff off a seed idea than start with a blank page…', that kind of thing.
When I started running the Black City campaign a month or so back, I planned to use random tables pretty heavily at the table, but decided that random tables were too slow for play. Then I dumped all of the random tables into excel and used various random generator algorithms to quickly build out content. It still needed refinement and mental elbow grease to put some meat on the bones, but I had a good skeleton for getting started.
Musings about "preparation versus inspiration" have cropped up on the blogs the past few days. Roger has an excellent post about the burdens of improvisation; the responsibility of making up content threatens to drive the DM towards balanced encounters, which risks a bland campaign. He muses whether you would you knowingly throw your players against unbeatable opponents made up on the fly? Noisms questions the zealotry of prep as the one-true-way; preparation is nice, but dropping the 4th wall once in a while isn't the apocalypse, either. Whereas -C is also firmly on the side of preparing in advance.
I'm certainly seeing the benefits of preparing more in advance. Geez, it can be a lot of work, but it feels good to have that stuff at the game table right at your fingertips. So let me tip a hat towards my wise compatriots; prepare in advance, even if it just means scratching the barest notes you need. Put your notes down in writing; pregenerated content is impartial and provides plausible deniability when crap hits the fan. "No, seriously, there's always been a dragon in that room." Improvise the details from your bare notes when you must. When you need to create stuff whole cloth on the fly, use the dice and random factors to avoid your dull personal biases. The dice are free to be arbitrary. The dice are free to be interesting.