First of all, you should only use keywords that are contained in your content, because this improves the search engine link popularity, so people will more likely find your content. The easiest way to figure out which keywords to use is by using the Keyword Weblet to extract the relevant keywords. Just paste the text of your content into the Keyword Weblet and click the "Run Query" button. The result box will show the extracted keywords. If your content is very long, you may have to do this in two parts (the tool can only handle about 1200 words at a time). If you wish, you can submit your content, use the Keyword Weblet to extract the keywords, then go back and edit your content to enter the keywords.

Another nifty tool on the Keyword Weblet is the Search Trends area. You can look at the hourly search trends to find popular keywords that relate enough to your content, that they can be incorporated. If you find any, be sure to use the full term or phrase in both your content and keywords.