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We are converting an old static html ecommerce site to a new platform. The old site has excellent ranking for some of the products. In order to maintain our ranking we will implement 301 redirects from old to new pages (as the urls will change to SEF). I am using Googles Keyword tool (in adwords) and entering each page url of the old site (there are hundreds, I'm doing the top 50 in traffic) and generating a set of keywords, then sorting each list by global searches. For each page, Google's Keyword Tool is giving me hundreds of keywords, but in meta tags there should be no more than 15, so I need a method to choose the keywords on the new page. Question:  in the new meta tags should we emphasize the most common keywords (as defined by most global searches) or the least common keywords? I would hate to lose the good ranking for the least common (long tail) keywords.