I simply talk louder than my children. When they start bickering/fussing I start yelling over them:
Be kind to one another, tenderhearted, forgiving one another, even as God in Christ forgave you.
Ephesians 4: 32
And then if they don't stop you just get louder and louder incrementally until you are yelling so loud all they can do is stare at you.
I have found this to be very effective in the van where 87.3% of their bickering happens.
But yesterday we were walking into the library and Child Fuss and Child Fight started in on each other.
Child Fuss suddenly remembered and stopped short and looked at me with wide eyes.
And she said pleadingly,
"Mom, please don't start yelling Bible verses at us in the parking lot!"
And all the fussing ceased.
And I smiled a little smile and we went on our merry way.