Remember the Book of the Month clubs that were so popular in the 1950s and 1960s? They are bigger than ever today with Oprah's book of the month setting sales records time and again.

Offer your own monthly special deal. Consider setting up a special offer that your repeat customers can take advantage of once each month.

A car wash business offered a free wash to core customers on a certain day each month. They lost money on the wash, but made it all back and then some when each of those return customers lined up at the gas pump.

The "once a month" marketing strategy is a great way to get old customers to buy consistently. Your special offer also puts them in a special club. Customers who think of themselves as people "who are regular customers" will be more likely to buy regularly. The way we think of ourselves has everything to do with what we spend money on.

Think of products or services that get used up and have to be repurchased. For example, I write press releases. It can be a good idea to issue a new release once each month.