Making snap judgments when you're tired or frustrated. Instead, take a deep breath, count to ten, and think about the message you want to send before you rush to punish.
Letting your kids compare your rules to your ex's. Nip this conversation in the bud by telling them matter-of-factly, "That's the way it is here. Dad and I just have different opinions about certain things."
Fighting with your ex over trivial things. It's not worth making a big deal about how much candy your kids eat or whether they take a bath every night. Save your energy for more important issues.
Giving in because you want your kids to love you. Trying to be your child's friend instead of his parent will backfire when you lose your authority.