Google Webmaster Tools provides a lot of great data. One of the things it will show you is 404 errors on your site. If you look at this data carefully, it can help you increase your valid in-bound links.
Of course, if you see 404 errors listed for pages that you are linking to from within your own site, you need to fix that right away. This will already help your site's overall page rank.
But the more interesting thing is when you see a 404 error for a page that someone else linked to. Most likely this occurs when someone links to your site, and then mis-spells the link (and also doesn't check it). When this happens you have people who gave you links for which you are getting no credit. Such a waste.
However, the fix is simple: Put in place a 301 redirect from the page that the person erroneously linked to, to the page they intended to link to. This is a great simple way to pick up link credit for those broken in bound links.
Google, Google Webmaster Tool, Site Links