Online user reviews are a great concept. On websites that offer them, you can read about the experiences other consumers have had with whatever you're thinking about buying, from a computer to a cruise vacation.

Like all great concepts, this one, though useful, can sometimes be flawed in its execution.

Part of the problem comes from "astroturfing." This is the practice of a company (or its public relations or advertising agency) planting positive reviews that appear to be from actual customers. Instead of the grass being real, it's artificial, like AstroTurf.

Little can be done about this trickery except to develop a discerning eye for it. If a review is glowingly positive, full of superlatives, don't automatically discount it; just be skeptical. It's the same with a review that's entirely negative-it could have been planted by a competitor.

A bigger problem than astroturfing, according to those who follow user reviews, is honest reviews written by actual customers that just aren't useful. Unhelpful reviews typically result from the fact that most people have little experience writing reviews and don't have the knowledge or background to add the context needed for the review to be as useful as it could be.

This can be solved fairly easily, and you can be a part of the solution by keeping a few simple things in mind when you write user reviews yourself, according to Esther Schindler, a professional freelance writer who has written more than 400 (free) reviews on Amazon.com.

This well-regarded web marketplace does a good job of helping you decide if any given review is worth paying attention to. It lets users review the reviews, and with each review, it includes the number of people who reviewed the product and the number who found the review helpful. Amazon then places the reviews that users find more useful ahead of those they find less useful.

In order to make your reviews as useful as possible, the most important thing to keep in mind, said Schindler through email, is to put yourself in your readers' shoes. Write for them, not for yourself. "Readers want to know if THEY are going to like the product. They're interested in your opinion only insofar as it helps them make a good buying decision."

Think about the types of users who will be reading your review. Depending on the product, some will be newcomers, some will have a bit of experience with the product category, and some will be experts with lots of technical knowledge. When relevant, try to meet the needs of each type.

Along with thinking about your readers, think carefully about the product-think beyond simply whether or not you like it. As an aid in this process, said Schindler, ask yourself these three questions:

• What does the product promise?
• How well does it achieve those goals?
• Is it a good value?

What a product promises and how well it fulfills that promise means talking about specific features of the product.

"Why" is a key question here. "Explain WHY you feel the way you do," said Schindler.

Along with what a product does and how well it does it, talk also about what it doesn't do-any features it may lack that you would have found useful.

Instead of interspersing what you like and what you don't like about a product, most readers prefer that you describe what you like first, followed by what you don't like, said Schindler. Don't just provide a long list of features but instead talk about what's useful or well-executed and what's not.

As common-sensical as it may sound, read the product's instructions before evaluating it. Sometimes certain features reveal themselves only when you do this.

Reviews are almost always better when you've worked with a product over a period of time rather than merely providing a "first look." A gee-whiz feature may get old quickly rather than be truly useful.

The best professional reviewers distinguish themselves by talking also about similar products on the market. Amateur reviewers won't always be able to do this. But if you can, even if it's only about a product you used before the one you're reviewing, it will make your review more useful. Tell the readers what is different, better, or worse about this product.

If appropriate, talk briefly about your experience with the product and why you're qualified to offer judgments about it.

Finally, be succinct. Get to the substance quickly rather than forcing readers to wade through a lot of introductory material. And when you reach the end, stop.