So you want to get your ex back? Actually, that's the wrong question, because you can 'always' get your ex back.
The right question is, how badly do you want to get your ex back? Easiest way to fix it is to think back over why your ex dumped you and make it right.
So if your ex dumped you was because you were didn't have a good job or because you weren't willing to meet your ex's friends and family, then get a job and go meet your ex's family.
And make sure you let your ex know about the changes.
Another magic word -- Sorry.
But not just a plain sorry.
You need to be specific about what you're apologizing for, and make it seem sincere.
You can back up the apology with your actions, and make sure your ex knows that you really, really are sorry about whatever happened and you promise that it won't happen again.
Ever.
Another thing you can do is make some changes in your appearance.
Physical attraction is a big factor in keeping relationships alive, and if you let things slide -- so to speak, then you're going to end up in trouble.
Start a fitness regime and wear some nice threads, and make sure you parade the new you in front of your ex.
Won't take much else for you two to get back in the sack, if your ex thinks you're hot again.
And lastly, you might want to consider getting some outside help.
You see, the reason you're not able to get back in the good books of your ex is because your ex simply cannot trust you any more.
So anything you do or say is bound to be met with skepticism.
If the same assurances and apologies on your behalf come from a trusted friend or family member, it might just do the trick.