Explain the story. The preface is most often used for this purpose. Give your story a quick run through and refresh your memory as to why you wrote it in the first place. Tell the reader why the story itself is important and why it was important for you, in particular, to write the story.
Make a list of all of the names of the people and organizations that may have helped or contributed to the story. If you make a list before you write the preface, you are less likely to inadvertently leave out names of people who were important to assisting in your work.
Choose your words carefully. The preface helps to set up the book and may influence whether a potential reader actually buys the book or not. The preface should not be considered as something that is extraneous material.