Alright so you have either recently been through a break up or you have been broken up for a while.
If it was more recent then make sure you don't make the mistakes that a lot of people make in this situation that really hurts their chances...
Mistakes like begging, pleading, calling too much, or just being very needy and desperate in general.
Things like that really push your ex away...
What you should be doing is showing maturity, confidence, and a positive attitude at this point, yes it will hard but it will also greatly increase your chances of getting your ex back when the time comes...

• If you are already passed the break up point and things ended badly, try writing your ex a letter and say that you agree with the break up and apologize for any drama that happened.
  
• Wish them well and just go on your way (You may not want to do this but it works, it shows a lot of independence and confidence on your part)

Reestablish Attraction This is not the easiest thing to do because a lot of times you can not see your ex personally or they are not talking to you...
A good way of rekindling attraction is to show your fun side (will be hard after a break up but it's in your best interest to do so)...

• Take mutual friends out and do something out of the ordinary, make sure they take pictures (chances are your ex will end up seeing them)
• Take a day trip to a local land mark or somewhere cool where you know your ex would like to go.
  Take pictures and post them on your Facebook page...