Not link BUILDING really, but...
Go to webmaster tools and look through the crawl errors for your site.
You will find crawl errors for pages on your site that don't exist. See how many links there are to those pages. Often, someone makes a link improperly to a page on your site and they mistype the URL, goolge follows that link to a non-existent page, and tells you there is a 404 error when it can't find a proper page as a destination to that inbound link.
create a 301 redirect to the proper URL.