Yesterday I downloaded from Google a CSV file containing the top search queries for my main web site, starting from July 2009. If you have a Google account, I recommend you to do that because it's very important for having a clear idea about how users find your site. Further, these queries show also the frequency of keywords during a certain period of time. For example, in July I got many queries about specific topics related to web development (particularly to CSS) but later the frequency decreased significantly and, with a little surprise, also my page rank (one point).
This is probably due to the fact that I didn't update the contents of my site for several months. Google loves fresh content, so you should plan content updates on a regular basis. But the big surprise were... images! Many people stumbled on my site during their Google searches just by typing something like "skeleton face"! So here's another point to consider: always use the alt attribute for images and a proper file name (this should include some keywords related to your content), obviously when convenient. Google, as always, is a source of big surprises.