Create a main character that is also a preteen. This will help the reader identify with the character. This character should have qualities that the children reading would like to see in themselves, such as courage, intelligence and determination.
Outline an exciting story in which the action moves quickly. Preteens are not going to sit still for long stretches of narration or exposition. In part because of video games, movies and television, preteens have come to expect things to move along at a breakneck pace.
Write your book, creating dialog for your characters that is humorous and logical, since children can smell it a mile away when they are being condescended to. At the same time, keep in mind that you are writing for younger people, so leave the thesaurus untouched and write using language that is accessible to them.
Familiarize yourself with the current slang used by the kids. Avoid any profanity, but use terms and expressions in your dialog that a child of that age in this time would actually use, unless your story is actually set in the past.