This is opinion, but I think 95% of the time, that when you break up with someone, you cannot be friends afterward. Most people would agree with that and it is some sort of rare, perhaps scripted, TV moment when it is otherwise.
Surprisingly though, we often find ourselves in situations where someone we dated is still in our life. Either due to mutual social circles, friends, place of employment (your own fault for dating at work, asshead) or hell… you still live together.
That last one is nuts, but New York City, where the rent touches the sky, you sometimes make tough calls. I’ve been through all of the listed above, so trust me.
You don’t have to become mortal enemies or frenemies with an ex, but it does not take a lot of rational thought to conclude that keeping a former lover in your day-to-day is a recipe for trouble. Whether things ended on good or bad terms, the best thing to do is go live your separate lives. In the case of the former you can likely be civil in inadvertently mixed company, like a party or dinner with said mutual friends.
If things ended badly, keep them in your mobile and on networking sites friend lists, so that you can ignore phone calls and avoid events. If they are not a life broadcaster, let your friends know the deal, so you can get warnings ahead of time for social events.
Regardless of what you choose to do, check with reality first. Reality will tell it to you straight that emotions are tricky and people are crazy, so you best excuse yourself from dumb situations before that girl you dated for two weeks attacks your wife.
Think I am kidding?