A lot of CMSs have addons for "share this". I use Drupal and IPB mostly and there's a plugin for Drupal, and the latest IPB has the share buttons built in. I'd say there's add-on modules for most CMSs.
Onsite: Make sure your keywords are in the title, description and keywords meta tags, make sure your keywords are spread about in the content. Offsite: Make sure your keywords are in backlinks from articles, blogs comments, social bookmarking e.t.c
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