This is a fairly easy tactic to implement on most sites, just go through and add the attribute rel="nofollow" to the links to low value pages in your navigation, and throughout the body of your site. When I do this I don’t nofollow the links to the sitemap or any links from the sitemap. It’s useful to pass a little bit of PageRank even to low value pages and the sitemap is the easiest way to do that.
If you want to get tricky, you can really direct the flow of PageRank through your site so the PageRank flows through a path through your site from the homepage. For sites with only one or two products / services you can nofollow links to all but those pages from the homepage and then decide where the PageRank should flow from those pages. For large E-Commerce sites, I prefer to pass as much PageRank as possible to the category pages, and then through the category pages to the individual product pages.
Remember, like all on-page SEO tactics, the rel="nofollow" trick won’t help boost your rankings or traffic if you don’t have a good link building strategy behind it. Don’t think that this, or any other on-page tactic, can replace good old fashioned link building.
It’s worth noting that even if you nofollowed all links to a certain page on your site, it doesn’t mean that the robots won’t crawl it; it just means that they won’t pass any ‘credit’ for those links. If you don’t want a page to appear in the search engines ever, you’ll need to do that through your robots.txt file.