-Be incredibly honest but be very careful about opinions. Most people interested in your review will probably want just the facts.
-Be very blunt...don't ever assume that the people reading your review understand all of your terminology. I suppose you could attempt to speak to your target market, but depending on what/where you're reviewing your target market could be vague or change often.
-Objectivity can help keep you out of hot water. Some people will look at any form of favoritism and just never trust anything you say again.
-Definitely go with Slaymate's suggestion, as well. Think about reviews that you've read that you trust and try to emulate them.
Best of luck to you!