I agree that the idea is secondary, as it usually changes. I'd really recommend you read Paul Graham's essay to understand his arguments that ideas are secondary, that co-founders matter a lot, and why you should/ should not start a startup.
After delving into these ideas, choosing the actual idea to pursue, I found that using swot (http://en.wikipedia.org/wiki/SWOT_analysis) analysis can be useful. Several questions from the application for YC are also useful (and can be categorized into Strengths, Weaknesses, Opportunities, and Threats) for fine graining: http://files.dropbox.com/u/2/app.html. Note that this application form is old, but the questions are still relevant.
As a getting started, you may found useful the summary I've crafted a while ago: http://metaphysicaldeveloper.wordpress.com/2010/04/30/scrapb...
After delving into these ideas, choosing the actual idea to pursue, I found that using swot (http://en.wikipedia.org/wiki/SWOT_analysis) analysis can be useful. Several questions from the application for YC are also useful (and can be categorized into Strengths, Weaknesses, Opportunities, and Threats) for fine graining: http://files.dropbox.com/u/2/app.html. Note that this application form is old, but the questions are still relevant.