Recently I ran a session as guest GM for a friend's group, but thanks to being loaded with writing assignments just before game day, I had almost no time to prepare at all. I also expected no more than four players, but ended up with eight, which later became nine. Thankfully however I had a bunch of adventure seeds in mind, some nicely proactive players, and some tools which aided on-the-fly GMing. The players liked the cinematic nature of the game, and were suprised when I revealed my prep time. Thinking back on what I did, I realized there were techniques there that other GMs may also find useful.
1) Have a Bunch of Adventure Ingredients Ready
We were playing a game of pulp air-adventure, where the players would be mercenary fighter pilots from the end of the Great War. So I dug into my mental library of pulp tropes and came up with zeppelins, air pirates, and the Red Baron.
The key here is to have a bunch of plot elements and visuals that you can throw together to create points of interest for your players. As long as you're familiar with the genre and/or the milieu of the game, it's pretty easy to come up with adventure ingredients quickly. Raid movies and other media for visual ideas. The innermost kernel of the adventure I thought up was the climactic scene from Flyboys, where you have this big swirling dogfight around a huge balloon.
2) Pick a Player and Hand Him the Ball
We kicked off the adventure by going right into one of the core elements of the pulp air-adventure genre -- a dogfight. To get the players into the spirit of things, I introduced one of the PCs as a senior pilot of the merc squadron, and the situation was he was there to test the rookies. This passed a goodly burden of the GMing load on this player, as all the other players were now responding to his cues.
In your games, you can select a player character as the lead-in to the adventure; find something about that character that will give him or her the motivation to get all the others involved, and set up the scenario accordingly. A side benefit of this is that the players will get more opportunities to roleplay with each other.
3) Give the Villain a Plan and Modus Operandi
Give your villain a goal, and a means of accomplishing it. The villain will execute the plan whether the PCs are there or not; the adventure however revolves around the fact that the PCs can either find out about the villain's plans, or get involved through the villain's execution of the plan.
In my adventure, the mock dogfight suddenly turned real when the squadron received an urgent radio message (I introduced radios, though a bit anachronistic, so the PCs could interact during combat) alerting them to a luxury liner zeppelin being attacked by air pirates. The villains were trying to force it to dock with their as-yet unseen zeppelin carrier so they could rob the rich passengers -- who were the cream of European society-- and hold them for ransom. The PCs thus had a chance to interfere with the villain's plan.
4) Take Player Cues and Run With Them
One of my players gave his character the name of 'Sir Guise' and roleplayed him with a British accent, so we agreed he was of the British aristocracy. When they landed with the zeppelin, the wealthy passengers came pouring out to thank them; I had one of the passengers recognize the aristocrat pilot and greet him by name. To my surprise, the player said 'Hey Edward, old chap, how're you doing?' Edward? British aristocracy?
I had said the game was set in 1926, so I quickly thought, Aha, THAT Edward! Imagine the players' surprise when I told them they'd just saved the Prince of Wales. This was not something I'd planned at all, in fact the idea I had at the time was to have a rich Italian-Spanish heiress looking for a sunken galleon in the Caribbean -- but hey, having the Prince of Wales made an interesting plot hook!
I hinted that the Vultur Squadron would be very interested in kidnapping this prize, and right on cue, the players cooked up a scheme to use the Prince's presence as bait. (It turned out the player wasn't thinking of Edward VIII either, but Edward just happened to be the first British-sounding name he could think of).
5) The World is Your Character
Think of the whole milieu as your character. Every NPC, every location, every object that the PCs interact with is just a facet of that mega-character that you're running. You can have it respond to player character words and actions in a fluid, lifelike way by coming up with details you know are appropriate to the milieu at any time because you know the milieu so well.
And just as individual characters have a purpose, so the world-as-mega-character has its own: to engage the PCs. Depending on how they interact with your world, the world can challenge them, provide clues and info, and spring surprises that twist the story of the game in wonderful new directions, as that player did with his casual drop of a name.