Brother MacLaren said:
One good compromise might be to try to get the PCs to decide what to do next at the end of a gaming session. You can give them several options, or let them come up with their own, and they can show you the consideration of giving you prep time.
Amen. Its not a railroad, its a highway system- and every juncture occurs at the end of the evening so you can write the next road at your leisure. You still have to define the available choices during the session, of course- which means you may have to make ad hoc rulings and then justify them later once you have the time. This is ok! In fact, no one ever need know.
DM: I present to you... the campaign climax!
Player: I have a rules exploit that lets us bypass this entire thing, and which also undermines the verisimilitude of the entire campaign setting because lots of other people COULD have done this, but inexplicably have not.
DM: That rules exploit won't work, actually. If it did, don't you think someone would have done it by now?
Player: Sure, if these were real people, but they're not. It is only in-game that this particular scenario has been tested against the best and brightest heroes of the past. Out of game, the only person who's reviewed this is you, and you probably missed this possibility.
DM: Well, you can try it, but don't say I didn't warn you. If you want to get everything ready for that, go ahead, but we'll have to do it next session. Its late now that we've wound up everything from today.
See? There you go.
And for those of you who have implied, or stated outright, that having these issues crop up means that I'm not a well prepared DM- screw off. If this doesn't happen to you it is because of one of three reasons. Either 1) your players accept the campaign premises and don't TRY to do this to you, 2) you enjoy campaigns in which heroes essentially hack the world for exploits (cool for you if you do, don't expect everyone to enjoy this), or 3) this DOES happen to you and you are lying about it online. I've yet to witness the airtight campaign, and I don't expect to, ever.