Tip #17: Using plotting to find a twist

Plot your basic story in a predictable manner ( ie the ending which most readers would expect)

Review key events or decisions characters make within the story and look past shallow perceptions to find a deeper, or darker motivation for those events to take place.

Keep thinking of misconceptions and different points of view until you come across one you like and then plot the basics of your story.

With that knowledge, review the ending and rewrite your sketched out finale.

http://writeanything.wordpress.com/2010/12/15/a-twist-for-your-short-story/