How can I be friends with my ex?

I honestly struggle with this question. If I had to think of a perfect situation, it would be where we simply did not have the same goals. Probably coming down to kids and marriage. If this person did not want those things, I'm sure I could potentially be their friend still. This would help if before we were in a relationship, we were friends first. That way, we would have had some common ground to start with. I'd also say that if our relationship was built on honesty and truth. If we fought and argued but at least we were honest about who we are, then it seems like there would be trust. Those things, in my opinion, are grounds to let someone go, but still recognize their value as a person, rather than someone that you were intimate with.