Right after generating new ideas a first valuation can be made with the help of a decision matrix using different criteria. Therefore the team, lead by the moderator, assigns the ideas to a prepared matrix. Possible criteria could be "Wow-Effect", feasibility, effort, goal oriented etc. Each idea receives points that correspond to the criteria. For example feasibility, a high value means that the idea is realizable, whereas a low value means the opposite. Instead of the matrix you can also use a coordinate system (XY) to which you assign the criteria. Using this method allows you easily to judge over the degree to which the criteria are fulfilled. For this method each participant receives sticky dots, which they can place within the coordinate system. One colour stands for one Idea.
If the ideas are written on Post-Its they can be placed directly on the coordinate system.
http://mindgroup.ch/blog/entscheidungsmatrix/
http://4managers.de/management/themen/entscheidungsmatrix/
http://www.mindtools.com/pages/article/newTED_03.htm