The number one problem we run into during the vetting process here at Indies Unlimited is a book’s description, also sometimes known as the book sales pitch or the book blurb. Too long, too short, too detailed, too vague, too too too, blah blah blah. What it comes down to is: many authors cannot write a book description on their own.
via Book Description Basics – Indies Unlimited
K.S. Brooks thinks it might be okay if a writer doesn’t automatically know how to write a pithy, industrial strength description for his/her book. We’ve lived with the manuscript for months, possibly years. We “know too much” about it to create the best 250 or 500 words of description the book needs to sell.
Her article on Indies Unlimited includes links to related how-to articles along with a list of considerations. If you’re publishing your books yourself or going through a small press that relies on you to write the description for Amazon and the back cover, this article will give you a running start.
–Malcolm
Malcolm's Round Table 
I hate writing book descriptions. I’ve found that, apart from the other great advice in this post, it’s a good idea to write the description, set it aside for a few days, edit it, set it aside for a few days, etc.
I don’t like writing them either. Setting them aside and seeing how they sound a week later is a good idea.