You've got three weapons at your disposal - your letter, your synopsis, and your sample chapters. They can't just be good, they have to be excellent.
Your covering letter needs to make the editor want to read your synopsis, the synopsis needs to make them want to read your sample chapters, and the sample chapters need to make them request the full manuscript. Here are some resources to help you do that.
Your Covering Letter
Your Sample Chapters
youwriteon.com – want to know the worst about your first 7000 words? Here's the place to find out. Honest, unprejudiced feedback from randomly assigned readers who rate you on all aspects of your writing.
Authonomy – Harper-Collins's online slush-pile. It will eat your life but you'll learn what you're up against and understand what it feels like to be an editor faced with thousands of manuscripts.