Apply the 80/20 ratio, i.e. identify the few major customers who buy 80% of your sales - therefore who pay the most.
The few major accounts
Get good credit agency reports to indicate the right level of credit. Get to know customers' payments staff. Visit them at intervals. Cultivate them to get priority treatment - as you would buyers. Give them priority attention on queries and service.
The mass of non-major accounts
Get the right individual's name for letters and phone calls, perhaps from credit application forms or routine correspondence.