Writing Amazon reviews is a great -and free!- way of getting your name out there. The trick is to create product reviews that people will actually like reading. Humans aren't always rational, and that shows in their ratings of bad reviews. In many cases, a negative review will be voted down, simply because of its tone, or more broadly because it only emphasizes negative points.
That's the first tip: if possible, only review books or products that you actually like. Not only will it make the review much easier to write, but it will avoid the problems mentioned in step 1. If you genuinely believe in the benefits of that particular item, it's a win-win-win situation: you get to write a review for something you're interested in; you are boosting a product you truly vouch for; and finally, you're not associated with the negativity of a one-star review.
Fiction books are much harder to review than technical books or nonfiction: for the latter, you can objectively measure how actionable or informative the book was. For fiction, your book review is just a summary of how the book made you feel, and the mental images it left behind once you finished it. So, for fiction, focus on the intangibles - the pleasures you derived from that mental journey, and possibly how well the author writes, or developed the plot (measurable).
One sure way to get good ratings for product reviews is to offer users choice: by adding product pictures and a video review, anyone reading will be able to pick based on their preference, and on the best format for the product. I remember reading a flashlight review: the top review, unsurprisingly, included an Amazon user showing the intensity of the flashlight in various lighting conditions. For that particular product, a video review was the perfect format.
The final, but possibly most important tip is brevity: only create a long review if you must. Some up-voted reviews are sometimes only two-lines long, but they were enough to give the prospective buyer the information he or she needed. Think about how much you love this site. Every topic explained contains the implicit promise that it will be concise: in five steps, sometimes even less!