I've been spending a great deal of time lately crafting the back blurb for Inquisition. Luckily, I'm friends with a number of writers who are great at back blurbs and they gave me some great tips which I will now pass on to you.
The three paragraph structure: This is one of the most common formats for romance novel back blurbs. It's an easy way for experienced readers to recognize a romance novel. There's one paragraph each for the hero and heroine, and then the final one for the main conflict of the story. Each paragraph should be short, no more than three or four sentences. Keeping the pacing of each paragraph quick generates excitement in the reader. If possible, end each paragraph with a hook.
The personality, job and problem character description: Book blurbs have to be brief and catch the reader's attention. There's a lot of information to squeeze into a short space. Giving the character's personality, job and problem can hook a reader and allow them to identify characters they would like to read about. For example: Exuberant toy-maker Santa Claus has an overwhelming management job and his chief Elf isn't making it any easier. Or Shy and quiet would-be solider Steve Rogers is desperate to impress the U.S. Army and fulfill his dreams of being a hero.
Judicious use of keywords: Readers scan blurbs for certain things that interest them: secret babies, marry-then-fall-in-love, shapeshifters, vampires, soldiers, firefighters, small towns, urban adventures, suspense, humour, the list is endless. Make sure that the blurb reflects the book's tone and clearly shows its genre and other hooks.
Edit, edit, edit: Don't dash off your blurb as an afterthought. Spend time making sure every word counts. Get it as perfect as you possibly can, then put it aside for a week and look at it again.
The blurb is like the first three minutes of a date, it's the author's chance to create a connection with potential readers and convince them to give him or her a chance. Happy connecting!