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1) Be objective.
Keep the facts straight. Write about elements you enjoyed, and the ones you didn't. Compare and contrast against similar games from the same genre or other games from the console's library. Try to pinpoint gamers who might be interested in this particular game. Save the rants and raves for your personal blog.
2) Use the "inverted pyramid" writing structurehttp://owl.english.purdue.edu/owl/resource/735/04/
It's straight out of the AP stylebook. It is taught in public schools and universities, and is used in just about every newsprint and magazine article you've ever read. Give them the straight dope. Dish it out right away, then give all the reasons to support it. Save the trivial stuff for later. It's not original, and it certainly isn't a narrative, but the inverted pyramid grabs peoples attention which is important since you're competing against the rest of the internet for reader's short attention.
3) Proofread your work
Other people can't be bothered to read your writing if you can't even do it yourself. Fragments and typos are distracting and take away from the point you were trying to make. Wait 10 minutes, an hour, or even overnight before proofreading. I'm always correcting my own stuff hours afterwords because I'm not satisfied.
Whatever you do, make sure you're qualified to review it. Don't be like this IGN reviewer who credit-fed his way through an arcade game, then complained it was too short. He didn't try to master it, and he certainly didn't get the true ending.http://www.ign.com/articles/2000/07/26/giga-wing