You can find plenty of books, blogs and other resources that mention good practices of data management, especially in the area of customer and potential customer lists. You should adopt some processes in your business when managing databases. We decided to concentrate only on three elements of that jigsaw.
Some methods we are mentioning further down are controversial; some are very well known and more universal. Whatever you decided to do, please make sure that the latest changes to the legal procedures that concern private data are not going to stop with an implementation of GDPR. There will be more regulations coming in, restricting access and tightening communication processes. Maybe some surgical approach to the management of your data affected by privacy policy regulations may bring you surprising, positive results. It is a good idea to have a clear plan for the future to avoid disappointment.