There is a nice old school SEO thread at HighRankings Forum on the old topic of ranking well for singular cases, when you are already ranking well for the plural version.

For example, you are in the tops of tops at Google for [buy widgets], but you want to rank well for [buy widget]. The thread goes over some SEO copywriting tips on how to help achieve your goal.

Ian McAnerin has a nice tip, let me quote it:

For the on page stuff, you can control how your content is written. It is not as simple as removing an S at the end of the phrase. You need to make sure to rewrite the content on the page to make sure it still reads logically and grammatically correct after removing the S.

The only issue is, then you may drop in your ranking for the plural keyword phrase. Personally, I have seen many cases where either the plural version brings in the conversions or the singular case brings in the conversions, but it typically is one or the other. Of course, both would bring in conversions, but in many cases, one version brings in the majority of those conversions. This is something you need to test and fine tune over time, like most of your SEO efforts.

Forum discussion at HighRankings Forum.