Most search engines will weigh these heavier in their ranking system. Use the plural form of your keyword. Use both upper and lower case spellings of your keywords. Use combinations of your targeted keywords.
People generally search using phrases containing two or more keywords.
Use meta tags. Most search engines index your site by the meta tags. These go between the tags in your html and will not show on your web page. View the source on one of the top ranking web pages for specific examples of successful meta tag use. Use an honest combination of your 10 to 15 best keywords.