Narration, with preschoolers, can happen almost without them realizing it if you choose the right kind of stories. Preschoolers love rhymes and repetition - use that! Preschoolers love to tell the story with you - encourage that! Find books chock full of rhymes and repetition and let them finish your sentences. Look for stories with a certain amount of predictability (counting stories are great for this). Before you know it, they'll be telling you the story!
Preschoolers are also (usually) very visual and very concrete. They love props. It never hurts to add a few props to help preschool narration along. Here are a couple ideas.