1. Be accurate. For example, it would be inaccurate to say how annoying it is that every character in the book has a name that starts with the letter V when, in fact, there are only three of twelve who do. Those readers who despise when an author shows just such a lack of originality might turn their nose up at a book that is actually quite original in the area of character naming, and thereby, a potential reader is lost to its hard-working creator. In addition, anyone who has read the book will know it is not true, most particularly the author, someone who, ahem, has been paid for what she/he writes, and you will simply look, ahem, foolish.
2. Educate yourself. If you have written a glowing review that the professional reviewers hated or if you have criticized a book that the professional reviewers loved, this is something you probably ought to know before you hit the publish button. When you claim a book is, for example, “predictable” and “forgettable”, yet, the professionals who are paid to read and review books for professional publications from which professional buyers make their professional book buying purchases claim the book to be “refreshing” and “unique”, even, "elevated above the genre's usual fare", in reviews that can be read online by anybody, (including your readers), you run the risk of looking ignorant.
3. Resist reviewing the author in the midst of your book review. No one is perfect, everyone has their own set of personal strengths and weaknesses, even people who write books, so keep the review to the merits of the printed page, not the character who wrote it. If you have a beef with the author, perhaps you are not the right person to do a review on his/her books. When you start to spout off with conjectures in the midst of a book review about the author, whether you know the author or, especially, if you do not, you simply come across as petty.
4. Remember that the author is a real person. With feelings, (and insecurities) (and possibly paranoia) and hopes for a successful career. If a book is truly awful, a reviewer has every right to say so—even if it isn’t. However, unless the author is so successful that your pithy little review (does that sound bitter?) is of no interest OR/AND, he/she doesn’t have time to read them all, the chances that the author is going to read your one or two star review filled with cutting remarks is highly likely. And when that review is nestled amongst plenty of four and five star reviews, you just sound cruel.
5. Keep your ego out of it. Most reviewers do a good job but once in a while there will be one who makes it his/her mission to write something witty and snarky in order to make themselves look, er, well, witty and snarky. Though this might be entertaining for your friends and readers, it really is a form of bullying; i.e. cutting someone down in order to elevate one’s self. In the end, this comes off as plain old jealousy.
6. Be respectful. After all, the author has published a book. This is no small achievement. In fact, most would say it is a big deal. A person has to be somewhat intelligent to get a work of writing between covers, as few books by stupid people make it into print. You don't have to like every book you read, or even pretend to, but do you really think you know better than a smart author, a professional editor and professional book reviewers? When you couch your negative remarks in condescending terms, you just might come across as arrogant.
7. Remember that books are a personal experience. How one responds to a book involves many factors. Try to be open-minded in your review approach. Of course, your review is your opinion and that is what reviews are all about. However, damning statements might turn away a reader who has different tastes than you and who might have enjoyed the book if read. You owe some amount of partiality (and accuracy and respect and all the rest) to those who read your reviews. If not, you simply come across as someone who shouldn’t be reviewing books.