Include singular and plural versions of the word and active and passive verbs plus common misspellings. Ask friends, associates and customers how they would search for your site. Put the most important words first in the list. Search engines will penalize you for too many repetitions of the same word, so do not repeat a word more than four or five times. Two or three word phrases work better than single words.
Many words have more than one meaning, e.g., Web pages, spider webs.
Use different keywords for different pages within your site. Meta keywords present you with a Catch-22. If you use many different meta keywords, the relevancy of a keyword is diluted because of the percentage of times it appears in the keywords tag. If you use few meta keywords, the search engine may overlook your site. My opinion is that you should use fewer rather than more meta keywords to improve your site's relevancy rating.