Pretend you are searching the web for sites that have themes similar to your site. Include popular synonyms of your keywords. Also include common misspellings of the keywords. List phrases of two or three words that might be used. These phrases are of special importance, because when people search on single words, they usually get more hits than they can examine.
People then enter phrases of two, three, or more words to narrow the searches.
If you place the same phrases in your meta tags, your site will more likely come up earlier in the searches. Make nouns plural to catch both singular and plural words by surfers. Omit spaces after the commas to reduce the length of the keyword list. Do not enter keywords more than once. Words in phrases should not also be entered as individual words.