Plan the blurb before beginning to write it. Be clear about the difference between a synopsis and a blurb. Though they have some similarities, they are different. An author usually presents a synopsis summarizing the plot and including the denouement when submitting a manuscript. Write a blurb to entice the reader by promising twists and turns without giving away the ending.
Write two to four paragraphs total, depending on the size and complexity of your book and your publisher's instructions. Introduce your main characters in the first paragraph or two. Set up the plot. Introduce the external conflict -- be it villain, plot or place -- in the next paragraph. Use punchy words, such as "gripping" and "wicked" to arouse the interest of the prospective reader.
Focus on the target readership. If you are writing a crime novel, for instance, do not describe the heroine's romance because your readers will not be primarily interested in that aspect of the book.
Read as many other novels' blurbs as possible. Recognize what language and hints make you want to read the book.