predictive pipeline

What is they key to accurate sales forecasts?  Sure, it’s all of the things I’ve written about before, like sales process, uncovering compelling reasons to buy, selling value, thoroughly qualifying, etc.  But haven’t you witnessed more than enough opportunities where all of that was completed at an acceptable level and the business still failed to close?  Don’t too many of those well qualified opportunities become delayed closes or losses?  So what is it?  What is the one thing that will accurately predict whether or not an opportunity is strong enough to pursue with everything you’ve got?