I have written several posts on Quality Score, but I recently found two articles on the subject that make it worth revisiting. Quality Score is Google’s way of assessing how relevant your paid search keywords are to the searchers you’re targeting.
This article by by Craig Danuloff discusses in monetary terms what the real costs of a low-quality-score are and this recent article, also by by Danuloff suggests 5 Steps to Improve Your Quality Score.