If you're one of many men wondering “what do I say to get my girlfriend back?” you'll be glad to know that there are many things you can say that will help. The really important thing is that she understands what you are trying to say. There is a lot of truth to the rumors that women here things completely different from the way men say them.
You might be saying one thing while she's hearing something completely different. Have you ever complimented the way a dress brings out her eyes only to see her looking at you horrified and accusing you of telling her it makes her look fat? This might be a somewhat extreme example but I'd be willing to bet something along these lines has happened in your relationship.
Women try to make their words mean so many different things while men have a tendency to say exactly what they mean. You may mean noting more and nothing less than what you said but that's rarely what she hears. It can be a bit confusing making the mental translation from one to the other.
Many times it isn't what you say that matters nearly as much as how you say it. If you're having trouble finding the right words to say to her consider putting it in writing. When you spell things out in black and white you can avoid saying the wrong thing in the heat of the moment (not that you'd ever stick your foot in your mouth but the odds are good that you both know how to push each other's buttons). It also allows you to make sure you say everything you mean to say without leaving anything out. More importantly, it gives you the opportunity to read it all together and see how it sounds before she gets the message.
Tell her your sorry. Acknowledge that you've caused her pain. Accept responsibility. Explain how you've changed. Ask for the opportunity to make things right. You'll be amazed at the results.
As you can see there are different options for different problems but if you're wondering what to do to get her back you should find that taking action to solve the problem causing pain in your relationship is a great place to begin.