Getting back together with your ex boyfriend isn't necessarily going to be easy, but I can show you how a fair amount of self-work and dedication to the cause can make it be a lot easier than it could be. If you're going to make him want to come back to you, you'll have to make some compromises and sacrifices, but that's just the give-and-take of a healthy relationship.
When a relationship fails, usually that's a good indication of something being wrong. If he left you, that something was likely one of two things: either the relationship was getting stale with not enough of you in it or it was getting claustrophobic with too much of you in it. Either situation here is relationship suicide.
Getting him back after he's run away like that isn't a picnic in a sunny park, but if you stop doing the things that made him want to leave you, you have a pretty decent shot of fixing your mistakes. Take a look at yourself and the relationship, and figure out what things that led to the breakup were your fault...and yes, a lot of it will be your fault. Not playing blame games, just saying that generally the person doing the leaving is having an issue with the person being left.
Really take a look at your situation, and determine what things that eventually led to the breakup were your fault and which weren't. Discard the ones that weren't your fault (since you can't really do anything about them) and focus on the things you CAN change, the things that you're responsible for. If your biggest problem was either of the things I talked about above, I may be able to offer some help.
If things were going slow between you and just drifting, make up your mind to do more with your partner! Go out more, spend time together, do things that are fun for both of you! A relationship shouldn't be boring, it should be the most wonderful time of your life! Don't let it get stale with inactivity, it's just such a waste of what could be such a good thing.
If you were having some issues letting him run his own life, try backing off a little and remembering that he IS an intelligent person who can think for himself without having you on his back all the time. We laugh at it in the movies, but a controlling woman is a nightmare in real life. Try not to be the number he doesn't want to see on his Caller ID, or the person he doesn't want standing in the doorway when he comes home. Give the guy some room!
So if he's run, take the time right after the separation to give each other some space...and for you to do this analysis I've been telling you to do. Once you've dealt with your problems and are "on the road to recovery," you can make contact with him again --but keep it light and simple! This is a fragile time, anything that makes him uncomfortable can drive the nails in the coffin lid of your relationship. Don't be too forward, just say something along the lines of "hey, just wondering how you're doing." Slow and steady wins the race.
And once you've been in contact a little bit, chances are good that you guys will start seeing each other face-to-face again. When this happens, it's all about doing things that the two of you enjoy...if possible at all, doing things the two of you used to really like doing together when you were a couple. It'll bring back fond memories, and possibly stir up some old feelings.
Always try to let the changes you've made to yourself speak for themselves without announcing them. If he simply notices that all of a sudden you're genuinely more enthusiastically into the relationship, or all of a sudden you're backing off a bit and letting him think for himself...it'll probably impress him quite a bit, and open a few doors.
From this point it's really all in how you handle things. I can't make a 100% promise that anything you do will work to make him want to get back together with you, but among your choices that give you the best chances, the things I've told you here should do fairly well towards paving the way to the healing of your relationship and the start of a new one.