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The help pages on SEO Moz will give you a lot of guidance as Keri says.  I'd seriously recommend making sure that where you can you ensure that your page gets an A or B by going systematically through the recommendations that the on page research section of SEO provides.  I've seen considerable gains from just making sure that a page ticks all the boxes listed to take it from an F to an A. On page Old stuff still has a part of play