There's always the danger to focus too much on attracting new customers while neglecting your old ones. This is a widespread and very dangerous game. You have to be able to balance the two because you don't want the customers who've stood by you taking their custom elsewhere.
To retain your old customers, always build strong relationships, offer the personal touch, give them a reason to want to stay with you by offering loyalty incentives, offer them second to none service and always be willing to listen to them, among others.