Remember you might be in constant hunt of new and new customers but your existing customers are your loyal customers who know you well and are right under your nose. Never overlook sales that one can get from the existing customers who have purchased something from you. You will get more business from the existing customers than purchasing on the new ones.
If you are not following your key customers you will be losing a lot of profit. You have already put in time and money in getting new customers and established credibility and cultivated a good relationship the very first time. So do not let them go.