1. Scope and change orders
Defines exactly what you are hired to do, and what happens when the job grows. Scope creep is not a personality problem, it is a missing clause: without one, every "could you also just" is free work you cannot bill.
The line to get right: List the deliverables, then state that work outside them is quoted and agreed in writing before it starts. The phrase "in writing" is the whole protection.