When you write mystery novels, a certain amount of planning is crucial. In my YA novel, Under the Family Tree, there is a murder, and as the writer I chose the murderer, the motive, and all related events that occurred before, during and after the crime. Then I mapped these all out in a timeline. Who knew what... when, where and how. Once that was done, I picked out a few tiny, innocuous seeming details and sprinkled them throughout the story. These are actually clues, but for the reader, they won't be so easy to detect. It will be like passing a person in the street that you once knew but don't immediately recognize.

Unless you pass them again, of course.