The Lovecraftian Approach To D&D
Lovecraft thought and wrote this way in his works: We're just a back-water collection of puny humans who have no clue about what truly lies outside of our own little corner of the galaxy. Every so often, people in his works would get a quick glimpse through the portal into those worlds and be changed forever.
This way of thinking has fundamentally changed the way I approach the game. When seeing it from this perspective, the DM isn't really creating a world himself, it's more like he's relaying it. That presents some interesting things. First of all, it means that a DM's description of it is not necessarily going to be precise, it's just his vision or interpretation. The DM thinks that it's a 30 foot drop onto the cave floor below - at least that's his best guess. He thinks the monster has 14 hit points. He thinks this merchant knows the real cost of the armor.
Now, that's not to say that this is a guessing game. The DM has a pretty darn good idea of what this world is about, but it gives him some flexibility. It's a great excuse to give to his players when he screws up a little.
Setup timelines and events that would really happen based on the layout of your world. If the castle lies next to the troll caves, it's likely those trolls would come raiding the castle and its surroundings quite often. They should be doing this with or without your players input to the situation. They don't just start raiding when the players arrive.
This is the mindset you should have before even planning to run a campaign. I call this site Tenebrous Tales to imply that these stories are not for us to create, they ought to be thought of as already existing, and we're just the messenger of it.