Your customers don't always tell you they're unhappy. Sometimes you only find out when they go somewhere else. You have to ask whether they are satisfied.
Customers prefer to work with friendly, positive salespeople.
When you stop calling on customers, they stop calling you for orders. Never rest on your laurels after a big sale. Use that energy to push onto more selling. Be available to your customers 24 hours a day. Never assume anything when you are speaking to a customer.