Most search engines will weigh these heavier in their ranking system. Use both upper and lower case spellings of your key words. More than 3 common forms of the same word will not help much. Although, common misspellings can help. Use combinations of your targeted keywords.
People generally search using phrases containing two or more keywords.
You must use meta tags. Many 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. Use a good combination of your 5 to 10 best keywords .