With spoilers for: The Sixth Sense; The Usual Suspects
A good twist is one that you could have guessed, but didn’t.
This is why so many stories fail in this regard. The perfect twist is predictable without being predictable. It is causal within the mind of the audience, and yet completely unforeseen. But how does anyone go about writing such a thing? And where do they find their ideas in the first place?
A Few Types of Twist
Twists basically work by subverting an assumption made by the audience. In some cases, this assumption comes from the form of the story. The Sixth Sense, for example, preys on the notion that ghosts look different to normal people on screen, so when Bruce Willis gets shot in one scene, then looks perfectly fine in the next, we presume he survived.