Keeping up with your customers is critical. If you don't know who's buying from you, or who's close to buying from you, you won't know who you're marketing to. Consequently, your sales will go down and your entire company will suffer.
Thankfully, there are tools out there to help you track information about your customers and maintain relationships with them. Chief among these tools is a content relationship management (CRM) system. You can benefit immensely from using a CRM.