Since (general) you are here, what does your prep actually look like (some of you have answered this already)?
I tend to fill a single empty page in my campaign notebook with sketched out ideas based on what is happening next (I am running a sandbox game and I make sure to let the PCs decide what they are doing, but ask them to tell me so I can prep at least a little). After that, or just before the session, I find maps and set up encounters that are likely to be needed (I run of Fantasy grounds, so this only takes a few minutes). If something unexpected happens, I tell everyone to take 5 and then throw together the required encounter and map during the session. This rarely takes more than a couple minutes because context led to the need in the first place.
When I am running a con game -- which are usually 3 or 4 slot episodic, ongoing games -- I build "the world" and the first "adventure" before play, then use the time between slots to riff off what happened and build the next. I usually have an arc in mind and a thought about what the climax is, but more often than not player decisions change that. I explicitly only run games at cons I am familiar enough with to improv and quick prep. I learned this lesson...