Write down what is included
List the deliverables by name, the format they arrive in and the number of revision rounds. Vague language such as ongoing support is where unpaid work hides.
Write down what is excluded
Exclusions are not hostile. They are how both sides agree on the boundary. Common examples include copywriting, third party licences, content migration and work after launch.
Agree on how changes are handled
State that anything outside the list is quoted separately before it starts. Then the conversation is about a new piece of work, not about whether the original promise was broken.