Co-parenting is difficult. It is immensely more difficult when approached with a negative attitude. Not only is it detrimental to your co-parenting success in practical terms, it can have negative effects on your children and their experiences with both parents. It is imperative that you refrain from bad mouthing your ex in front of your children even at the most frustrating of times. Seemingly harmless questions like “why didn't your father pick you up from school timely?” places your children in the position of defending one parent to the other. If you have questions about how or why your ex is behaving in a certain manner, ask them directly.
Follow your parenting plan. If you ex spouse asks for an extra 3 hours with the kids one Sunday afternoon and you are not inconvenienced by this, feel free to oblige. However, keep in mind that straying from the parenting plan is a slippery slope into a world of miscommunication, boundary crossing, and disappointment. While you may agree to variations from the parenting plan as a means of building good will, it is unlikely that you will be able to successfully cash in on that when it comes your turn to make an exception to the rules.
Focus on your common child rearing goals
It is very likely that you and your ex have fundamentally similar parenting ideologies. While that may seem hard to believe, it is probable that underneath all of the disagreements about the specifics of parenting, you both prioritize things like education or extracurricular activities. Focus on these common goals when resolving conflict. Acknowledge your ex's point of view. Simply showing someone that you are listening and you understand that his or her position is merited goes a long way towards finding suitable compromise.