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A long time ago on a former site, I identified about a dozen key searches and kept a record of the position of each in G, MSN and Y. I then worked on improving the ranking of each term that was "below the fold".
That site is long gone and on the current site I haven't even checked the PR. I regularly check the position on real life searches from the site stats. As a result of that I identified a key search that wasn't performing, made some minor adjustments to content and pushed up visits by about 30% (from an admittedly low base for a new site).