Should I optimize for the plural or singular form of my keyword phrases?

Both the plural and singular form of your keyword phrase are searched upon often according to Wordtracker. Which one should you choose?

Both. Keyword phrases such as the singular and plural forms of your words, which are related, are extremely simple to use within the visible copy on your pages. When we naturally write or speak, we use both plural and singular forms of words, so why restrict yourself to one or the other?

Never optimize a page or a site for just one keyword phrase. That is the equivalent of SEO suicide in my opinion. Every site has hundreds if not thousands of phrases that someone might use to find it. Figure out what they are, and then use them to your advantage to bring highly targeted visitors to your site.