Remember that a good plot twist can't be thrown in at the end just to fool the reader. You have to play fair. When readers look back, they should be able to see all the clues and realize that everything fits into place. With a perfect twist ending, the reader ought to smack their forehead and say, "Wow, what a great ending! I should have seem that coming."
If you kept the reader guessing until the final confrontation, your twist was exceptional. Here are some tips to help achieve that goal: