Official Google Webmaster Central Blog: Specify your canonical
Basically, in the <head> section of your pages, put in:
<link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish" />
to tell search engines that whatever variation of this content they're seeing on the CURRENT URL (with session params appended, affiliate tags, etc.) is really located at http://www.example.com/product.php?item=swedish-fish. Handy if you were doing lots of noindex/nofollow/301 madness to funnel search engine respect to your canonical URLs.
WordPress plugin to take care of the details for ya:
Canonical URL links - Yoast - Tweaking Websites