Increasingly, search engines and directories are evolving into search portal sites that combine both methods of searching. They're also incorporating search technologies intended to make searches more relevant and user-friendly, and to eliminate spam. As a result, search optimizing must take into account a growing number of factors. To optimize a site so it can be found, you first need to determine what words people will use in searching. This is important both in optimizing for search engines and in submitting to directories.
People usually search in groups of words or phrases.
It's extremely difficult to optimize for a single word search. There are a number of resources for seeing how people actually search online. MSN's bCentral has a new beta keyword suggestion tool. Compare the list you've compiled with results from these, and boil it down to 20 or 30 prioritized words and phrases that might be used to find your site. Though the major search engines all differ, the basic on-page elements they consider are generally the same.