Don't use keywords that are unrelated to one another.
Don't use the same keyword twice.
Don't repeat any word in the keyword phrases more than three times. For example, you should avoid using "dvd", "dvd player", "dvd burner" and "dvd recorder" as your keywords because the word "dvd" repeats more than three times.
Don't fill your description with product or brand names that you don't usually have in stock (known as "keyword spamming"). Spamming keywords could affect your page ranking or even cause your Store pages to be removed from search engine indexes.