Be polite. Swearing indicates that you lack intelligence while insults show you lack self-control.
Be brief. A five-thousand word essay is not a review. Keep it to less than 100 words.
Explain. Something like " This sucks, so there ." shows you cannot marshal your thoughts coherently and can thus be ignored.
Get an account. You will not be taken seriously if you do not show you have taken the time to join in properly.
Back up your stance. Do not accuse others of bad writing unless you can describe exactly what the problems are and suggestions on how to solve them. Making grammar/spelling errors in your reviews while doing this will lead to you being laughed at.
Think carefully before posting. Others only know you through what you post, don't make them think you're a self-righteous moron with delusions of adequacy.
When I Review: Well, when I review, it's never anonymous. I always put in a kind word for the author. As a matter of fact, in all my years of Fan Fiction I only said something bad and negative ONCE. I'm like the opposite of Simon Cowell. Still, I try to be honest. Critisizing a story is something I do through advice. Reason for always having something good to say is because when I review a story, it's because I liked the sumary and it captured my eye, therefore leading to me to reviewing the story, and adding it to my favorites when I do. Further, I usually review every chapter of the story to boost the amount of reivews ther author gets. However, going through October 17th, 18th, and 19th of 2007, I was forced to let everything out and actually flame somebody. Name won't be mentioned in public. I'm better than that. Still, it's all settled now and I don't have to flame anymore, unless I am once again forced to. I only flame people when they flamed somebody I know, of if they flamed me. I never act first. I'm happy to know the three days of warfare ended though, lol.