In today's market it is most important to be up close and personal with your customers ... they are far more receptive to communications and promotional material which is specifically created for them, to meet their needs. This process may not be as difficult as you think as you probably have the basic requirements in your customer and sales databases or your computer's address book.
It may be as simple as a letter, addressed to them, mentioning their company name and a product they have recently purchased from you - which all shows you have taken a personal interest in their purchase. It can be as complex as offering each of your recent purchasers related products and services with interchangeable images, details and prices.