In a recent discussion of Robin's Laws of Good Game Mastering, ColonelHardisson suggested for, well, roleplaying tips. I enjoyed this little tidbit from an archived issue:

How To Twist Plots
From: Max B.

Hello Johnn,

I have some "tricks of the trade", and I want to share these.

My style of GMing isn't creating plots/stories from scratch, but taking existing ones and twisting and turning them ad nauseum.

There are (in my experience) six methods of plot tweaking. Let's take a simple adventure outline ("evil wizard kidnaps a princess") and see what the application of those methods will give us.

"As is"
It's just a basic plot with cosmetic changes (e.g. names of the wizard and princess, method of kidnapping, place where the captive is held). Okay, it isn't a tweaking per se, but creative changing of details can make interesting adventures. Must be done with caution though -- it can become boring after several repetitions.

"Upside down"
One of the major plot elements is changed to its direct opposite. Maybe the evil princess somehow locked the wizard in his tower (and must be in the same tower to keep him locked); maybe the wizard didn't kidnap the princess, but instead rescued her from a terrible death, and so on.

"HOW MANY of them are here, you said?"
Too many, actually. For example, ten or so evil wizards compete with each other in an attempt to capture one princess; the evil wizard captured not one, but many princesses; last month there were multiple captures of princesses by evil wizards, but only one is Really Significant (tm).

"Bait & switch"
Imagine the wonder of the PCs when they discover that something is absent in the story: the wizard didn't kidnap the princess, he's just deluded that he did. Or the kidnapped girl isn't a princess, but her female bodyguard is (16th level fighter capable to escape on her own, by the way, and very bored and angry because she has orders not to); or (for a really complicated twist) both wizard and princess are impostors --she is a cleaning maid and he's a wizard's would-be pupil (and where on earth are REAL princess and wizard?).

"Amateurs, damn amateurs!"
Something's gone terribly wrong. The wizard's servants were so lame that they lost the princess soon after kidnapping. Now she's somewhere in the wilderness/city slums/Astral Plane, and nobody knows that!

"For King, Country and sheer fun of it"
Humourous story twists are good, if done properly. Probably this tweaking method isn't so great when used alone, but it is when used in conjunction with other ways... For example: what if the wizard kidnapped many girls and ancient custom dictates that the savior must marry one of those he saved, and only one PC is noble enough to be considered eligible for royal marriage?