One trick I like to do is to listen to my players roleplaying discussions between themselves and npcs. I do this because I enjoy roleplaying but also because I like to take parts of their discussion about "what if" and use it in our games. Players can be a suspicious lot and one great thing I have found is to play into that suspicion a little bit and in so doing the players actually have some control over the world with just their comments. If used sparingly it will give both you and your players a great satisfaction. Many notable npcs are created this way in my games on the fly. Sometimes, the dungeon really does have that mirror pool with the fire wand.
You can even incorporate the idea into the roll aspect, for instance I had a player ask me what he knew of trolls for the upcoming battle. Now I recalled his earlier discussion with another player that they roleplayed beautifully in which one suggested the trolls from the forest where these came from were a dark bluish color and were immune to fire but water hurt them. Now when he made his roll and scored well I rewarded him by telling him the exact thing he just said to his buddy in game. A smile parted his face and the following encounter was moved to a nearby river where they setup an ambush using their knowledge. I had never intended on using trolls at all and was going to use something else but because of the players passion it changed the story.