just as a witness to my parents' divorce. they tried hard to be friends, really didn't work for them.
Sometimes there just shouldn't be an effort made. My ex and I have remained friendly by not contacting each other.
Never be friends with ex's? Never is an extreme word. It was the right answer two out of three times for me. Never forget that true friends are rare, I wouldn't throw one away just because he or she was an ex.
And if that causes problems with your current partner, consider that you may have again chosen badly.