Title your review so that someone can see immediately what the review is about. Use descriptive phrases. The title should reflect the substance of your review, yet make the reader want to read what you have to say.
Give a synopsis of the story--without telling too much--in the first paragraph of your review. Explain a little about the main characters and hit the highlights of the storyline.
Write a comparison of the story to another of the same genre or by the same author. This is much easier if the book is one in a series. If you are being critical, make sure you have sound reasoning for your criticism.
State your opinion clearly in your final paragraph. A good review is critical to both the good and bad parts of any story. It will prove helpful to the reader's decision about purchasing the book.