Twitter response:

When you do blow it—and you will.

If you do lose your temper when applying discipline to your child and end up acting like a child yourself, you need to cool down, breath, and then sit down with your child and apologize for your inappropriate behavior.  Your child needs to see you admit wrong when you are wrong.  When your behavior is not appropriate ask your child for forgiveness in handling the situation poorly.  Allow your apology to be a teaching moment for your child.