Prioritization - never an easy thing
Scott Anthony of Innosight wrote a great blog post: 3 ways to prioritize a long list of ideas which provides some good insight into how to start.
I'll add to that:: I also try to think of contrasts. Good projects (and what made them good) vs. not so good (and what made them that way). Good customers vs. not so. Great opportunities vs. things you wish you hadn't done…and eventually you can begin to discern the patterns that make some things desirable and others not. That can eventually lead to something as disciplined as a scorecard, which I've written about in various books. Should I put a sample on the tools page? Let me know….