1. Be sure to include the GOOD, too.
It is so easy to focus on the negative, but you should both start and end each review with something that you find to be a positive about the item you are reviewing. Something honest, an insincere compliment won't help.

2. Make all comments constructive.
Criticism and suggestions are inevitable in reviews. Make them constructive, and with a positive tone. And don't pile on too much at once. If someone receives a hugely long review with all kinds of criticism, they are likely to be overwhelmed. If need be, offer what you believe are the most important, and then offer to review again later.

3. If they don't know who you are, give a tiny bit of information about yourself. I don't mean a novel, but I mean introduce yourself. You wouldn't walk up to someone on the street and start having a conversation with them critiquing their work, and not say hello. (At least, I hope not.) So, it is a good idea to go ahead and do this in the review, as well.