A hypothetical example of a keywords meta tag is:
<META NAME="KeyWords" CONTENT="star,trek,deep,space,nine,Skybox,trading,cards,card,nonsports,non-sports, trade,trades,collecting,card collecting,collections">.
It is really easy to get carried away with keywords, and there is a length limit to the keyword meta tag for most search engines. The problem arises when we start considering word tenses, phrase searching, and capitalization. It is best to just try and use the major words that you believe people will use when they search for your site and list them all in lowercase. People usually use phrase searching when they get too many results, or when they have a reason to look for a specific phrase. Only a few of the search engines provide case sensitive searches, and most people actually usually perform searches all in lowercase, even when it is a proper name (Sullivan, Capitalization). These problems aren't as bad as they may appear, for more information, see the complete article on capitalization by Sullivan.
To validate the types of searches people perform, it is actually possible to watch searches being performed on the Internet. I have identified two such places where you can eavesdrop on people using search engines. The first site is http://www.metaspy.com. This site lets you see random searches being performed on the MetaCrawler search engine. There are two flavors of this spy, one filters out obscene or inappropriate words, the other provides a true, uncensored sample of the searches. The second site, voyeur, allows you to spy on the Magellan search directory in the same manner. Its address is http://voyeur.mckinley.com/cgi-bin/voyeur.cgi. The voyeur search is unfiltered and may display 'inappropriate' material.