The right attitude to get your ex girlfriend back will depend on the girl and the situation. But there's only a few attitudes you need to have to cover all the situations.
The first case is what I'll call the "princess" case. This is the woman who knows what a prize she is, and she wants a man to treat her that way. Situations like this are rarer than they used to be. If you have a case where the lady doesn't think you were good enough for her, then you're going to have to charge in on your white steed and sweep her off her feet. Think as romantically as you can. She wants a proposal on bended knee, and nothing less will do.
Improve your work and financial standing as well as you can, too, since this is the kind of woman who requires high maintenance. Before you go to all this trouble, double and triple check to make sure that she REALLY is worth it, because this woman has to be "won." Don't fall into the trap of the kind who has an inflated sense of her value.
The next case we'll call the "judge." This is when you really had a perfect match, but you blew it big time. You got abusive, you cheated, you lied and ran around, or various other offenses. In the judge's case, you have to "serve time," "make restitution," and most importantly, show her that you're a reformed character and you're ready for a new life together. Your success or failure, along with the right attitude to get your ex girlfriend back, will depend on your sincerity in turning over a new leaf, and her willingness to forgive you.
The last case is one we'll call the "kitten." Now the kitten's problem is that she was looking for a big bad wolf to come along and be a manly man's man, and instead she sees you as just another kitten who happens to be male (though she can barely tell). This is the most common case in the present day, and it's getting more common, as a generation of men and women raised in the "politically correct" decade have discovered that sensitive, soulful, syrupy men make lousy lovers. The kitten is more concerned with sex and her attraction to you than other types.
The strategy with the kitten is to man up. This is the only scenario where it's safe to try to get with a couple of other women for the short-term, as a sort of rebound therapy. But this time, throw away your Star Wars figures and your manga collection, grow some chest hair, win a couple of bar fights, pump some iron, get yourself fixed up, and come back like a rock star. One who knows damn good and well that he's a hot hunk and she's crazy to pass you up.
By now you're seeing that most relationship break-ups fall into one of these three scenarios. Pick the one that matches most closely, or mix and match if you have a special situation. Then succeed, using the right attitude to get your ex girlfriend back.