Love your kids with everything you've got. Let go of your ex, as best you can, and focus on the impact your love is having on your kids. Stay true to that value and you will always choose the high road.
It's all about the kids. If you've still got a beef with your ex you need to get over it. There's no point. You might have disagreements about stuff, but those should be handled with the same intensity as a convenience store clerk.