My experience, Tara, corroborates Kevin's wise words. I'm only friends with one of my exes because friendship was the defining feature of our relationship. With my other three exes it was either a more purely romantic and sexual relationship until we came to the conclusion that we were/had become two completely different people, a conclusion that did not always end well for us. If there was no friendship in the mix during the relationship, then chances are there's no potential for it in the future.
But even then, a friendship with an ex can only start after several months of minimal contact after the breakup, ideally when both parties involved have found new love interests. One reason I am best friends with my first ex is because our friendship began when we both started seeing other people. Now, in fact, I'm going to be a groomsman at her wedding this summer, as she's marrying another good friend of mine.
That's right, I'm going to be the only other person at that wedding beside the groom who's fucked the bride, and he knows it.