Marketing on the WebAs I mention on the SEO page, key word data is very important for determining navigational structure of your website, writing copy, and building your SEM campaigns. There are several good sources for phrases, I look a web logs, Google Analytics data, Webmaster Tools, and the words most often used on other high ranking web sites in the industry.
If you are looking to expand the words in your SEM campaigns you should also examine the “long tail” of your logs. What you will find is that some of the more specific phrases that occur less often (and are therefore in the tail of the data) actually turn out to be good conversion phrases, and because they are very specific, can be inexpensive to bid on.