I have been looking all over the web for an article as good as this one – he covers areas such as canonical tags but also URL parameter handling which is a difficult and complex area to find information on. If your site has been built in php it is likely that the URL structure will include strings like product_id and Colour_id. These may well have been changed by mod rewrite to shoe Google friendly URL’s such as www.widgets.com/bosch/drill-bits/p1045 – but the original non friendly URL can still exist.
By using URL parameter handling you can tell Google to ignore them.
The article is by Chris Soames and is here at Smart Insights: http://www.smartinsights.com/search-engine-optimisation-seo/index-inclusion/seo-avoiding-duplicate-content/