Read the novel from front to back. If you can't read it all, then read as much of it as possible. You can't summarize what you don't know, and a knowledge of the book is vital to writing a good blurb.
Focus on the first few chapters: the opening act and the conflicts it sets up. Ask yourself what the central dilemma of the plot is, and what challenges it sets forth for the main characters.
Analyze the central hero and/or heroine, and how they enter into the plot. They serve as the audience's surrogate, and the blurb should contain a sentence or two discussing their traits and pertinence to the story.
Consider where to leave the audience hanging. The blurb needs to set up the book's conflict, but not its resolution; for that, the reader needs to buy the book and start reading. Think about the best way to hook the reader and pique his interest once the basics are established.
Write up a draft of the blurb, covering the basic information and perhaps some additional material on the author. Keep it short and succinct: eliminate unnecessary material and allow the text to flow smoothly rather than getting bogged down in detailed descriptions.
Let the blurb draft sit for a bit--two or three days if you can afford it. The time gives you a sense of perspective and objectivity that helps with the editing process.
Revise the blurb two or three times: polish any rough spots, clean up grammatical errors, and drop any phrases or sentences which you feel are unnecessary. If you have a friend or an editor available, have him look it over and provide you with constructive criticism.
Finalize your blurb, check for any last-minute copy editing mistakes, and send it to your editor or publisher.