Try to incorporate singular and plural cases of words, as well as active and passive verbs. Also make sure your keyword list includes both general and specific words related to your site. You rarely want to target a single keyword, because, with the billions of words indexed on the Web now, one word simply won't cut it. Always use phrases, not plain words, in your list. Some search engines even give precedence to text near the top of a Web page, so make sure you place the most important stuff at the top.
Some search engines consider header text particularly important.
If your site mainly consists of images, you should use the ALT attribute to describe each image. The overuse and repetition of keywords may result in a lower relevancy score and possible omission from some search engines. Most search engines count only the first few occurrences of a keyword or phrase, and some even penalize you for repeating words to improve your site's ranking. Infoseek and Lycos are two search engines that penalize sites that are suspected of repeating keywords, and Altavista will disallow URL submissions from those who spam the index. Some search engines penalize your site if a keyword is repeated more than three times.