[exalted]

I run two completly different types of campaigns one for when the group is just out for a bit of lighthearted fun which can be quiet railroady (prepared more or less from first to last step) and high on combat encoutners being more game less roll play. But then that type of game has been decided and everyone knows this is lighthearted fun and monster slaying doing the quest is what we are going to do today and everyone is into that.

Second the more common and usually long winded campaigns I usually do a lot of background work on setting, imporant, NPC's, potential treasure if that kind of game, stuff that can happen and is happening on the larger scale. Then I set the players loose and se what does happen and how they mess up the setting. Some events will run in the background even if they don't choose to take part the world around them is alive and changing. But prepping more then very short plots (1-2 sessions) and usually ones indicated the PC's intend to follow these days seem useless, the player are making the story as the GM I usually provide options and twists not the complete path. Even so prepwork for many but perhaps not most sessions take much longer then playing them.

Someone above mentioned re-using what you have done with different names when it does't come up the first time. I also do this and it is a great way to ensure that at least 50+% of prep time is useful.