How do you maintain the element of surprise? People expect things to go a certain way. These expectations you can play on, secure in the knowledge of how things will actually go.
Do not reveal your secrets until the proper time. Plot twists are woven into the very fabric of a story, but it is important to hold back a few. When a reader thinks they know what you will do, it is the opportune time to shake things up.
The movements of a story are not arbitrary. If something happens, there is a rhyme and reason to it. On some level, the author must be aware of this. However, reveal as little as possible to keep the anticipation strong.
Fear not the savvy reader, who picks out the pattern before the work is complete. There is also an excitement in discovery, so long as a certain amount of effort was required to descry your design. And that which is clever cannot fail to impress.
Be willing to take risks. Modern readers have seen just about every trick in the book. An author must be cagey to elude the astute and confound the jaded. The genius is not in the gimmick, but in the timing.
Subtlety is key: it unlocks many doors. The tried and true will always outdo the desperate and new. Surprise is not in the idea, but in the execution. And it always sneaks up on you…