How to get an ex-girlfriend back for real?

It took a year to get over a bad break-up before I was my usual happy, positive self. And when I finally got there, a new girl entered my life and took notice. All was great, then a few months later she returned to her ex, and I feel I was part of the cause. What real-world, practical advice can one offer to get back with an ex? There must be some things that work and some things that don't. There must be some ladies willing to outline what a guy would really have to do if he was serious. Anyone?