Each project represents a work effort with a corresponding price. If you're working on something complex, try breaking it down into logical components, and creating a single, focused project for each.
One common problem that arises in freelance engagements is that the client and the freelancer have different ideas about when the work is completed.
Try to describe the project, including any deliverables or other end results of the project, in such a way that minimizes the chance for disagreement about when the work is done.
Focus on measurable results—e.g., a mockup re-design of our current homepage, or a collection of at least twenty photos depicting everyday people in rural China.
Creative work is inherently subjective, so you'll need to decide who, as between the client, the freelancer, or a third party, should have the final say as to whether the work is satisfactory and complete.