Research the agent and/or editor to whom you are submitting your manuscript!
People, I cannot stress this enough. How much time and energy you put in BEFORE you hit the "Send" button can make a world of difference between a publishing contract and a rejection letter. Never mind the fact that you should probably pay an editor to review/copyedit/develop your manuscript a bit more for you (let's face it, none of us can see all our own flaws, and our friends are often a little too kind to point them out, either). Acknowledging that agents and editors have specialized genres/areas of interest is more than just smart for you to do for yourself. It shows respect for the agent/editor's time and that you are invested in the success of your book (and they should be as well).
Seriously, people, if you wrote a non-fiction book about family law practice, don't submit it to a Romance novel editor (Yes, I've seen this done). There's no chance that your book will fit into the scope of that editor's publishing. All you end up doing is forcing the editor to waste their time rejecting you. That kind of character development is just plain unnecessary for you to have to endure.
The more you research and understand the nuances of the genre you are writing in and match it to the genres/areas of interest of the agents and editors you submit to, the higher your chances of success. If you really want your book published, give it the best chance you can! The world is not holding you back. You can be the architect of your own success!
Now, get out there and hire some freelance editors (me) to help you sculpt your manuscript so that when you submit it you really are putting your best foot forward!