I just started using service agreements this year. They are pretty basic. The first page outlines the services I offer with a place to put in a price for each one the customer wants. at the bottom of that is the payemnt options and a place for the customer to sign. Page two outlines some basic terms and conditions such as when invoices will be sent and when payment will be due, what I will do when I am there (mowing, trimming, edging, blowing), when service times are (April through November for me), late fees, and this line that protects me from customer negligence...
"The understanding exists that the ultimate responsibility for the health of a customer's lawn and landscape lies with the customer. Project Green Lawn and Landscape will not be held liable for damaged turf or plant material resulting from improper watering, adverse weather conditions, or other circumstances outside of the control of Project Green Lawn and Landscape."
Hope this helps.