It is important to keep in mind all of the good memories the two of you shared together, this will make it easier to transition into friendship. By focusing on the good times you allow for a positive, constructive environment.
When trying to develop the friendship it is essential NOT TO SLEEP WITH THEM. I know it’s comforting and familiar, but that will dredge up past feelings and just make it harder on the both of you. It can also cloud your judgment and give you a false sense of hope for getting back together. If one of you starts to develop more than platonic feelings for the other at any point, you need to take a step back and re-evaluate what is going on.
Some people do eventually get back together but this is not advisable when trying to first develop a friendship. Clearly defined boundaries are the only method you can maintain a genuine friendship with a past lover. You need to keep in mind that your relationship ended for a reason.
Honestly, you do not always have to burn the bridge between you and your ex. If you have overlapping groups of friends, remaining civil with your ex is beneficial, so if you run into each other there is no awkwardness. Relationships that ended as a result of cheating or deception are different than simply realizing you two are not compatible anymore. In situations such as these it is inherently more difficult to remain friends.
The idea of remaining friends with an ex sounds like an ideal situation, but the road it takes to get there is filled with obstacles. You need to work actively on it so that both parties feel comfortable with the new direction the relationship is taking. You must be prepared for what you might face and how the two of you are going to deal with it.