The project manager is responsible for controlling a project's requirements. To start the process of managing requirements the project manager works with the team to create a Work Breakdown Structure (WBS).
A couple of things to keep in mind regarding a WBS are:
A WBS should identify the level tasks to be completed, and relate to the project’s deliverables.
The customer(s), project sponsor, and stakeholders are actively involved in creating the WBS.
The WBS helps avoid future "scope creep".
As you can see the WBS is an important project artifact. The WBS accomplishes several things:
It assists the project team to identify and create specific work packages
It is another way of communicating the project's objectives to the team
It is the foundation for future project planning and activity sequencing
In closing, a WBS summarizes deliverables, shows work relationships, helps the team to estimate costs and perform risk analysis, and assists the team to identify project assumptions and dependencies.
The WBS is your friend. Start taking time to create one for every project.