Most Helpful Guy
You can't 'get' them back if they don't want to come back. If they realize your worth and what they're missing out on, they'll come back themselves. Nothing much you can do.
Most Helpful Girl
When the relationship is over, sometimes it's so difficult to accept, life is so different without that person and the pain is so unbearable that the only thing we focus on is to get him back, so that our life can be fixed and be like before. Unfortunately, things don't work that way and it's not a good idea to spend time and energy doing this.
Believe me, the only thing that will ever get you moving on is to focus on yourself. If you start learning something different, doing an activity that you've never done before, joining groups, going to the gym or to the pool and improving your body, hence boosting your self esteem... you get the idea. This will also allow you to meet new people, which is a plus.
Aside from this, leave your ex alone, don't look for him. And you know what? If you start doing all these things, looking more confident and active, chances are he will want to get back to you. But by then, you'll realize you don't really want him anymore ;)