Read the book you're reviewing actively (mark it up). If you've already read it, but aren't able to recall much about it, re-read it with a highlighter in hand. Mark sentences that stand out as clever and insightful; look for repetition of certain phrases or ideas. In quality writing, repetition usually indicates importance.
Ask yourself questions as you read. Consider what the author's arguments are. How does he go about making them? In other words, consider carefully, as you read, the book's style, themes, and thesis.
Note points and arguments that could and should be dealt with at length in your paper. Make notes by hand or in a word-processor document.
Put your material in a logical order. Cluster similar points, and find logical ways to bridge all of them to one another. Make an easy path for yourself from one point to the next by knowing how the paper will flow before you ever write it.
Open a word processor and commit yourself to writing the paper. Resist every urge to procrastinate; you'll have the most success writing the paper while your mind is still processing the book you've read. The paper may require less effort now that you're secure in your knowledge of the source material, but discipline is still necessary to produce a strong review.