Do your homework. Read widely on the topic and take copious notes about what other credible authors and theorists have to say on the subject. Remember that you have to refer to other sources, so note the citation information as you read, and save having to reread to find the page number for a quote.
Avoid phrases such as "I think" or "in my opinion" and try to dove-trail your ideas with those of the experts. For example, "Concurrent with suggestions set out by Anderson (2011), I agree that more work needs to be done in this area."
Support your opinion with quotations and data from other sources: scientific studies, books, articles and online materials. Line up the theorists who agree with your position and use their work to support your ideas. Also mention opposing ideas to balance the essay.
Write a draft of your critical essay and leave it for a day or two. Correct any typing or grammar mistakes. Also keep an eye out for commonly misused words such as "affect and effect" or "past and passed."
Read your critical essay aloud to yourself. Put yourself in the reader's place and determine if you are convinced or swayed by your presentation. Also, listen to the academic tone of your critical essay and edit accordingly.
Summarize the author's purpose. State the main purpose of the work and give an overview of the facts and ideas. Give your reader a clear idea of what the writer proposes when presenting her ideas.
Evaluate the writer's theory. Not all theories are created equally. Review the references used to support the ideas and evaluate the strength of the theoretical position.
Critically examine the work in terms of presentation of the material. Critically review the structure of the work and formulas of the ideas and theories discussed.