Natural backlinks can also be built by commenting on relevant blogs. Reciprocal linking can also go a long way in building your natural backlinks.
What I have found useful is using the website: http://promotingyourblog.org I have been able to find websites that are related to the topic I want to have to have a link on and in some cases be able to add a link right away on that site. I have also used it to find blog posting, found links on those postings that were broken and contact the web master of that site and offered links to my own websites with related information.