There can be many reasons for a relationship to break down; whether it be a cheating spouse, the addition of children, finances, abuse issues, etc...
Whatever the reason, there are 3 factors you will need to build on if you want to win the fight to get get your ex back.
Commitment: This will be required of both parties on some level, although it is normal for one party to seem more committed.
As long as there is some level of commitment on both sides, there is a much better chance of the relationship surviving the separation and you winning back you ex.
The best way to prove your relationship commitment to your (ex)partner is to simply make it/them your primary concern.
Try to use your time together to connect in a positive way by doing things together.
Good communication: Real communication with no name calling, blaming and so on.
You must let them understand how you feel without losing your temper so think carefully before you speak.
And the best way to get your ex back may simply be to pay attention and listen to what they are trying to tell you.
Often we hear, yet we do not understand what it is someone is trying to tell us; so care enough to pay attention and it will show.
Also don't underestimate the power of the casual conversation in a relationship; sometimes it's the little things that count.
Proper counseling: Good counseling can help you to win your ex back; or it may help you both to decide against trying to reconcile the relationship instead.
Either way good counseling can be very beneficial in the long run, especially if there are children involved.
Relationship counseling is generally best provided by licensed family therapists with whom both parties will feel comfortable.
Always look for a marriage counselor who is a licensed mental health professional.
Check with your health care provider, employee assistance programs, state or local agencies, or clergy may offer qualified recommendations.