The first thing a costumers does, after seeing the cover, is reading the blurb; or with an eBook the description. If this text is not convincing it is very likely that they will, rather than adding it to their shopping cart, look at the next book and forget about yours.
To avoid this, it is most important to write the blurb in an appealing manner. Having the following tips in mind will help you to achieve this:
Nobody likes Spoilers
Would you read a book of which you already know the entire plot or even the ending by just reading the blurb? Probably not. The clue is to draw the interest of the reader towards the story but at the same time not to give too much information. Leave some room for speculations and stay away from to detailed information.
You probably have already read different blurbs of various bestsellers. Have you noticed that most of them have a build up with a similar concept? First the reader is made familiar with the protagonists and the location, after that they are introduces to the happening, flowed by a plot twist which ends with a cliffhanger. This makes the story especially interesting and the reader wants to know more.
To the point
Please make sure that your blurb isn’t too long, about 200 characters would be perfect. Be as specific as possible. Do not use filler words and work with short sentences. Every single word has the purpose to arouse the reader’s curiosity. Having this in mind your words should be well chosen.
If your book is part of a series make sure to point out previous events but make sure not to give too much information. You could add some reviews from already released books, adding to this if you won an award mentioning this here isn’t a bad idea.
What wouldn’t be a good idea is to mention things which your friends or family said, this could appear unprofessional and have a negative side effect.