Define the Workflow Statuses - For a new customer requirement to move from an idea to completion, it takes steps and approval processes.  Workflow is normally tracked via the "status".  Let's create a simple workflow, where the client requests a feature, approves or rejects it, your team completes the requirement and bills it.  Below are some possible sets of statuses (workflow) for this process.  Notice that we numbered the workflow so that they appear in a logical order.
 

Workflow Statuses: 01-New (Awaiting Design) 02-Design Completed 03-Awaiting Client Approval 04-Closed (Client Rejected) 05-Approved 06-Ready for Billing 07-Closed (Completed)

Flowchart the Workflow - Flowcharting the workflow allows team members to understand the process in full.  We created the flowchart below using Microsoft Word.
 

Advanced Workflow - In our example above, we used simple workflow.  However, if your team uses software to manage customer requirements, you should be able to implement more robust workflow.  For example, the software should allow you to define "state transitions". This identifies how a status can transition from one status to another.  In our example, above, you may want to only transition from 03-Awaiting Client Approval to either 04-Closed (Client Rejected) or 05-Approved, because none of the other statuses would be applicable at that point in your workflow.  Likewise, the software should also allow you to define what fields (or items) you wish to make required upon different states. In the example above, if the requirement is changed to 02-Design Completed, we may want to require that the engineer enter an estimated cost.  Robust requirements tracking software will allow you to define the field attributes for each state transition.   Software Planner (http://www.SoftwarePlanner.com) does this nicely, you can see how this is handled from Software Planner by viewing this movie:

http://www.pragmaticsw.com/GuidedTours/Default.asp?FileName=Workflow