I look at and consider killer question: What questions if answered NO should stop the project. Set up and define these question at the beginning of the project so that all members including the sponsor are in alignment. And these questions should be asked through out the project and at key alignment points, not just at the beginning and the end.
By my definition the root questions are simple and the answers can evolve through the work done which should be symbiotic
Do people want/need it?
Can we / should we make it?
Can we make Money on it ?
This helps me to organize what projects have the most value and a foundation to provide WHY or WHY Not i am advancing the work. I also do not allow my self and only my self to answer the question but have subject mater experts pulled into the team to help ensure we are looking at things. (it is amazing when i am giving a NVP that assumes we will obtain 85% increased market share… Thus i challenge our business…) This can help prevent zombie projects as well.
Not gonna say this approach is taken by everyone at our company but i try (sometimes i loose)