It is often very hard to win back an old girlfriend, especially if she was the one that broke things off in the first place. If it was you that broke things off, it may be easier as you may only have to apologize saying you made a mistake but if she broke up with you, she must have had a reason and so before you can make any progress, you will have to learn what that reason was.
Often it will be because the girl thought you were not taking the relationship seriously enough and so there was no romance left in it.
If this is the case in your instance, you are going to seriously think about becoming more romantic, tell her you want to take her for an intimate dinner of for a secluded picnic somewhere where the two of you can be alone to talk about the chances of you having a romantic future together.
If this seems to be working you can then suggest a romantic weekend away together. Once these romantic rendezvous start though, if you want the relationship to last, you will have to keep up the romance side of things.
Although getting romantic often works in these situations, other popular approaches do not. One of those approaches is to beg the girl to come back to you.
This approach may work to get the girl back but the relationship will not last as having seen you begging, she will have lost respect for you.
Another approach that may also work initially is to threaten either her or yourself. This approach is also doomed to failure though as the girl will have returned due to either fear or pity, neither of which are good feelings to start a meaningful relation with.
Whatever approach you do try though, recent research suggests that you should never bring up the past, regardless of how many great times you may have had.
The reason why the research suggests this is because as well as reminding her of the good times, it will usually also remind her of some of the not so good times, one of which may have been the cause of the break up.
As it is so hard to get an ex-girlfriend back, before you start to even try you should seriously consider your decision. Is it the girl herself that you miss so much or is it just a case of you missing female company.
If it is the latter, maybe finding a new girlfriend would be the better solution; it certainly would be the easier one.
Remember, even though your chat up lines may usually work well, an ex-girlfriend will already be well aware of them and so may even be immune to your initial charm.
Something new is therefore going to have to be tried and that something new, is going to have to be good if you want a result that works well enough to not just get her back but to also keep her.