A useful technique adopted in agile is the MoSCoW technique. This stands for Must have, Should have, Could have and Won’t have this time. The MoSCoW technique is particularly useful when the project has many requirements and possibly a strict time scale.
Following the MoSCoW technique, the requirements are broken down into level of priority and then assigned to their MoSCoW group. For example, a high level priority requirement would become a MUST have requirement. It is highly recommended that you conduct the prioritising of the requirements with the business/client. This allows them to feel empowered in the decision making process and assists in encouraging them to think more deeply about the project requirements and whether they are in fact needed.
Learn more about the MoSCow technique at http://bit.ly/MoSCoW