After you've sent your direct mail, you need to track your responders and add them to your house list. Your house list is a list of past and current customers. Update the list as soon as possible to eliminate error.
Regardless of the mailing list usage you select at the time of purchase, if a prospect responds to your direct marketing, they become your lead and you can continue marketing to them at will.
Look for patterns that show different demographic categories responding better to different offers. You can use this information when you begin your next direct mail campaign.