We invoice again with a big red OVERDUE stamp and call. Then call again. And again. Then send a letter and another invoice. Call again. If there are enough outstanding accounts we sometimes hire a stern-voiced contractor to call people.

Worst case, we give a collections agency the extremely delinquent (like, years old) accounts. They basically split whatever they can collect with us. They usually don't get much (if people were gonna pay, they'd have paid)

Oh, and of course we don't continue to sell to people who are more than a few months overdue.