Your natural instinct is going to kick in when you want to get your ex girlfriend back from another man and this typically leads to a lot of crying, begging and drama. Most men have no problem badmouthing their ex's new boyfriend. If you do this to your ex girlfriend she's going to feel even more distant from you. If you try and convince her that he has a huge array of unflattering qualities, she's only going to see that you are the one who is immature and selfish. That type of behavior may actually backfire on you and make her feel closer to him because she'll feel the need to defend him to you.
A much more effective approach to get your ex girlfriend back from another man is to accept it. When a woman starts up with a new guy after breaking up with her boyfriend she expects her ex to fly off the handle. If he instead acts mature and accepts that she's moved on, it changes how she views him. You want her to feel that you want what's best for her. If you can do that, you stand a much better chance of getting her back.
The final thing you have to do is establish yourself as a good friend to her. Women love staying friends with their exes after the relationship ends. Take advantage of this and use it to maintain close contact with her.
Do it in non-threatening way so she doesn't feel as though you are trying to reestablish a romance. Lend her a shoulder when she needs it, but make sure she understands that you're only her friend. Once you do this, and her new relationship ends, she'll turn to you to pick up the pieces and that's when you can start rebuilding the closeness that you two once shared.
The things you do after your breakup will either set the stage for a reunion with your girlfriend or will ensure she's gone for good. Saying or doing the wrong thing can impact your future with the woman you love. If you still need and want her don't leave your future with her to chance, there are ways to win her back.