Before you start work on a project make sure you seal the deal in writing. For short one-off projects you could ask the client to send you a purchase order, but for any substantial piece of work you should use a contract for services or a 'Statement of Work'. A Statement of Work defines what project-specific activities, deliverables and timelines you are agreeing on with your client.
This is important. A contract helps you keep track of the initial agreement as the project evolves, prevents scope-creep and provides a legal framework in case anything goes wrong. It also defines your employment status, in other words that you are your own boss and not an employee. For this to be valid, make sure the way you carry out the project is consistent with what it says in the contract.