Read the book or article that you will be reviewing more intently than you would read most things. You will need to understand it thoroughly to write a convincing and effective review. Read it at least twice before beginning to write. You will find that you gain a deeper understanding of the author's intentions on the second reading, when you have the perspective of knowing what is to come.
Take copious notes on all the aspects of the work that you will cover in your review. Pay attention to the content and the structure, and write down any insights.
Gain an understanding of the context of the work by reading others by the same author, so that you are able in your review to understand the work's place within the author's body of work.
Write the review for the reader, not for yourself. Remember that your job is to help the reader to understand the work. While you may have opinions about various aspects of the work, don't allow them to overwhelm a fair appraisal of its contents.
Guide the reader through aspects of the work that may not be clear. You probably know more about the author and/or the work than the average reader. Rather than simply recounting the plot, examine the motivations of the characters, the historical context and the symbolic underpinnings of the work, clarifying those things that the reader might have missed.
Give the reader somewhere to go at the end of the review. Rather than answering every question, end the review by bringing up further issues for consideration, or by directing the reader to related works, either by the same author or on similar subjects.