A lot can be learned from the marketing strategies of Real Estate agents. When an home owner lists their house with an agent, that agent sends post cards to all surrounding neighbors to advertise the house listing.
This is something you could do when you sign up a new client to service. You could send postcards to neighbors in the surrounding area to promote the homeowner's lawn at 15 Main Street is now being serviced by you and your company.
Another thing to consider is to split the marketing costs with a realtor. When they send out their postcards that tell the local residents of the homes they sold, you could also have your lawn care business promote a service on the flip side.
Maybe the realtor could give you a ringing endorsement on the postcard.