Define in detail the services that you will perform for your client. Be as specific as possible. Frequently, if your client refuses to pay, the client will argue that you did not perform the services you were obligated to perform. Specifying the services will help you prove the exact services that you did agree to perform.
The services to be performed may be defined in the body of the Services Agreement, or, if it works better, in an attachment to the Services Agreement. They may also be defined on a matter-by-matter basis-using task orders to set out a particular set of services and change orders to modify the list of services.