Last night I spoke at the Agile KC (Kansas City) meeting. I was talking about the challenges of moving to agile when you're in an environment that seems content to do things using traditional approaches. I enjoy these meetings because it's a good blend of folks coming to learn and experienced people sharing with the group.
One idea that was brought up had to do with measuring value. The technique involves assigning value points to each feature or user story by the product owner. For example, the most important feature may get 10 points, some minor feature only gets one point.
As features are delivered, you track the points to show how much value is being delivered. So if you look at the graph below, you see for the first few iterations, the value goes up pretty quick but then it levels off. Since you're not delivering much value by the fourth or fifth iteration, you should consider ending the project and moving on to another project that would be adding higher value.