Regularly review estimates, deadlines and milestones to check they are still on schedule. If necessary, update the plan with new estimates and tasks and ensure that remaining tasks are necessary and estimates are still valid. Circumstances can change during a project but question every change and always refer back to the original business requirement as a corss-reference.
The document will form the definitive description of the aims of the project and, as such, can be used to manage the expectations of the stakeholders. It will also include acceptance criteria that will ultimately be used to judge whether the project was a success.