I improvise everything but NPCs stats...
The truth is: I improvise everything, except for NPCs and monsters stats. I have absolutely no time for doing adventures in advance, so I took on the habit, and now being lazy and seeing it works, I never bother myself to prepare something.
Well, sometimes I think about the next adventure (usually when on the way to the place where we play), and try to remember the good idea I got. Also, for a particularly important adventure I once made a quick sketch of a temple. On the other hand, I write down all NPCs, and prepares their tactics in advance.
When I come to the game session I have several Dungeon magazine and sheets of paper with me. Then, when I don't have any interesting idea on the spot, I pause; behind my screen I pretend to re-read my scenario, often opening a magazine in front of them, so I can get time to invent something. Of course magical items are also improvised, and I write them down thereafter.
There is something great with this improvisation method: every adventure has a duration of one gaming session only, and players appreciate that. Then, as nothing is prepared I can easily follow players' suggestions. That is: players think there is a plot to uncover and make various hypothesis and deductions. I pick the ones that please me the best and build the plot on the spot based on what they said. I can tell you: players are extremely happy to see how clever they are!! :-D
If you want to see the result of this improvisation, our gaming sessions are recounted on my campaign website (see "adventures" section):
http://www.dcrouzet.net/d20highlands/
You will see I pretend to use Dungeon magazines adventures. But it is not the case, even if sometimes I take some inspiration from some of their plot ideas. I wrote this down so players would not know the truth!!
Otherwise, I once asked on the FanCC mailing list if others did also improvise, and I was horrified to learn that almost all of them did!! There was even a DM who also improvise the monsters stats and abilities on the spot!
DOM