I have to admit that in my time as a GM, I probably haven’t put forth the sort of effort that many might expect. The thing is, that I think I’ve done pretty well for myself and my groups by weaving the story as we go. That said, there are things that I’ve come to learn are very important to consider when running games by the seat of your pants.

• Take Notes
  This should go without saying, but I’ve often failed to take adequate notes. This means that in long-running games where there are extended gaps between sessions, you run the risk of losing track of important details or useful plot hooks. Continuity counts when creating a world your players can sink their teeth into. Also, it’s one of the ways your world can seem more fair than it might actually be… 😛
• Stay Vague, Until You Can’t Anymore
  It’s long been understood that any engaged group of players regularly comes up with more dire circumstances for themselves than their GM ever had in mind. Keep this always in your thoughts as you build toward where you think you want the story to go. Listen to your players’ character dialogue, and you can transform the jump scare three encounters into the smugglers’ hideout from a randomized event into a callback to something the ranger muttered about when the party first set off into the woods.
• Play Dirty
  I’ve often heard complaints from GMs about how their elaborate plans for their players demise were foiled by a decision to take the walking tour through town instead of the train full of cultists. When you’re making it up as you go, you by definition don’t have this problem. Let the players build the gallows, just keep the rope handy for when they step onto the platform.
• Play to Lose
  With wha I just said in mind, remember that this isn’t a competition. Your opposition is supposed to give way, not to the players, but to the inexorable, inevitable force of the plot. Failure isn’t only not fun, it’s counterproductive. So, turn those failed die rolls into opportunities to make the story interesting. After all, who’s to say what was supposed to happen if that trap was triggered, you and only you.
• Act Like You Didn’t
  And keeping what I just wrote in mind, don’t ever let your players think you’ve given them anything. Reward them with more challenges. They’ve just overcome the cavern mazes of the Northern Highlands: reward, the sword that makes defeating the dragon that makes its lair at the heart of the subterranean labyrinth even remotely winnable. If you can sound like you’re disappointed they got it, that’s all the better.
• Keep the Spotlight on a Swivel
  Because you’re scenario isn’t foreordained you can easily adjust your narrative to allocate attention to whichever character has sort of been standing in the back, running things remotely. Everyone likes their time at the center of things, and keeping generic encounters for your tech guy and face in your back pocket. Waiting for the insertion of the right flavor text can do wonders for party dynamics.
• Be Prepared to Get Very Specific (or Cinematic) at the Drop of a Hat
  Practice describing things. Look around the room you’re in right now. Is it small? Is it well lit? Is it a train carriage with three rows of four seats split lengthwise by a narrow aisle with the California countryside slipping by the windows in the early afternoon glare? Does the masked assassin take a shot to the shoulder, or does her shoulder explode in a spray of blood and bone as the vector’s slaughter accelerator fills the air with a cloud of needle-sharp projectiles. Just because your sketching in the picture as you go doesn’t mean it can be without interesting details. In fact, adding just the right details to trigger your players imaginations can make for an even more memorable scene for each of them than a long paragraph read from a published module, even if every single one is seeing something completely different.