Don't take your existing customers for granted, or your existing customers may become someone else's new customers. If you don't treat them well, you can bet someone else will. Think of your current customers as if they were still unsold potential buyers. They deserve just as much attention as new customers do. Simple things like returning their phone calls and staying in contact after a sales will improve your chances of keeping your customers from deserting you.
Treat your customers well and give the best service, will increase customer satifaction. Customers deserve the best services you can offer, they are afterall keeping you in business.