I think the biggest difference between published and unpublished writers is perseverance. That doesn't only mean pitching your work to lots of places. Even more important is sticking to your writing, learning as much as you can, and paying attention to any advice that you're lucky enough to get from an editor, agent, or other pro. Writing is like any skill -- you can get better at it with practice. Keep at it and eventually you'll see your work in print.