Instruct your workers to also have a customer service mentality. If they are working at a site and the customer comes out to talk to them, they should stop working and talk to that customer. If a sweet grandma wants to bring them lemonade, drink it! When you go onto a persons property every week, you become a trusted part of the home. So behave like you are part of the family and that bond with the customer will serve you well.
Also be on the alert for anything you can do for the customer that is above and beyond the call of duty. Never miss a chance to do something for free for customers that you are building a long term relationship with. It may be no more than cleaning up a mess around the trash cans or nailing a few boards up on their back fence to keep the dog in. Those little acts of generosity will endear you the customer and build that sense of trust that results in long term customers who recommend you to friends and neighbors alike.