Co-parenting with an uncooperative ex-spouse

How do I handle conflicts with my ex-spouse over different parenting styles? Our divorce settlement stipulated that we would share responsibility for the children. The arrangement sounded fine on paper, but it's a disaster in practice. I'm a firm believer in structure and discipline. Unfortunately, that goes out the window every time the kids spend a weekend in the home of my ex-spouse, who cares nothing for rules or guidelines. What can I do about this?