A common way to prioritize (and it is nothing revolutionary) is to draw a graph with one axel for impact and the other for effort. It is then easy to find what to do. To make this easier I usually find one idea that I put in the center of the graph, and I then ask the group to compare the rest of the ideas to this one.
• Will this idea have a higher or lower impact?
• Will it require more or less effort?
When there are more ideas on the board, then I use the same questions until we find its place on the board. Then we take the next one and so on.