I was recently in the same situation and this is my game plan;
First off she broke up with me, she said 'we were too different!'
I told her I think we should wait a month before we see each other again.
In this month (which I am in now!) I've been going to the gym, learning Krav Maga and Mastering the art of seduction. My thoughts are that I obviously didn't demonstrate enough S & R (Survival and Reproduction) Value around her.
It's generally difficult to get back with a girl because she already KNOWS your S&R Value so in reality you have to convince her on something she already KNOWS. Essentially you have to change for the better and increase your S&R Value.
In the meantime i have kept in contact with all of her friends, I'm trying to pwn them for obvious reasons, pretty much next time you see her you have to walk into a room and be the guy everyone wants to be and every girl wants to be with. Not easy but I like a challenge
NOTE: The downside to what I am doing is that as I am learning the art of seduction I am feeling less and less attracted to me ex, not necessarily a bad thing but if you really like her I suppose it would be otherwise. I know of so many people that have gotten into the game to get their ex's back and have found that after a couple of weeks of Playing the game they don't feel attracted to their ex anymore, and generally become alright PUA's