But who is your twist is trying to fool?
Sometimes it is the reader. She is reading along, thinking one thing, and then, BAM, there is a shocking revelation and your reader gasps. This occasion is usually followed by the reader thinking back over the story and kicking herself for not seeing it coming.
Sometimes it is not the reader -- it is a main character. Maybe your reader knows who the villain is but your hero does not. Now your hero is blindly trusting the bad guy, and your reader is biting her nails or yelling at the hero (something along the lines of "No, no, no! Don't go with him! He's only going to...!"). Your reader sees things coming.
So if you have a big secret -- something you are going to reveal later in the book -- you have to decide who gets to know early. Does the hero know? Does the reader know? Do any, all, or none of your side characters know?
This is part of what I am deciding for BB right now. Twists -- who knew?