How to prevent going back to an EX!

The best  suggestion to prevent relationship regression is that you do the following:
As soon as you break up build an emergency kit containing:
DVD of real tear jerker movie like Beaches or Always or The Notebook
Frozen tuna casserole
Pack of cigarettes
Bottle of Cheap wine
Hose to run water on your windows like it is raining
Candles to replace the lights
Some very emotional music to play. Usually country music is best.
Keep this kit in a readily accessible place.
At your first sign of a serious moment of weakness: Pull out the emergency kit and do the following:
Give yourself a couple of hours to half cook the tuna casserole, put the movie on, light and either smoke or
just let burn the cigarettes,then put them out in the half frozen casserole.  Drink some cheap wine while you have the hose running on your windows, with the music playing and the movie running too. Wallow in the sadness of the entire scene until you are hysterically laughing then go do something fun. Like stand on your balcony and yell next!

This absolutely does work to get you over the maudlin self pity where you are wallowing.

On a more serious note:
First and foremost having a coach to support you through a breakup is the smartest thing anyone can do.  When you are  having a moment of weakness remembering what the good times were like, no matter what time it is, I have you call me first.

One of the things I have you do when you start working with me is create a toleration list . (That is a list of everything in your life that you are tolerating from a burnt out light bulb to a significant other cheating. ) So the first action I would have you do is reread your toleration list and consider how you would be settling to go back.

The most important thing you must  consider when thinking about calling an ex is WHY?
READ the CARD you have previously typed and printed and placed by your phone that says
1. why did you end it. ?
2. what will it cost you to call and compromise your integrity ?
3. Does he/she deserve to have any part of you?
Also you would have made a list of things you can do to nurture and pamper yourself  (I call it taking extraordinary care of yourself), at the same time that you made your toleration list.
So right after reading the toleration list I would have you do one of the things on your taking extraordinary care of yourself list.
The idea is to remember why you are not together and do something to cherish yourself.
When you are ending a relationship, it is essential to have actions set up to avoid the possibility of regression to the good parts of your previous relationship.  If you are not completely committed to ending the relationship, you will revisit it until you are hurt again. So once you have made the decision to end it, you have to have a serious action plan for those moments of weakness.
Additionally its a good idea to delete all contact numbers, emails, Facebook, Twitter, and other possibilities of contact when you break up so that you are not tempted to use any of those things to connect.