Marketers call it the 80-20 rule. It's the peculiar phenomena where 80 percent of your business come from 20 percent of your customers. But there's more to this than meets the eye.

People  who spend a lot of money with you obviously like your kind of products or services.  This means that they may also spend a lot of money with your competitors.

With a sensible mailing plan, you can convince your best customers to give you a greater share of their spending. Moreover, if you use your records to understand these customers, you can create a profile of them. Then create a mail program to attract more customer who fit the profile.  They will become good customers, too.

You might want to consider using a computer database for customer information.  There are many off-the-shelf programs available and they make it very easy to analyze information and send personalized messages to the people in your files.