Natural link building, which really implies not doing anything proactively with the intent of building links, that any link obtained is too many. What I mean is that if you specifically write an article with the intent that it will bring in links to your site, then that intent is wrong according to the search engines. What search engines want is that people should read that article, use that tool, buy that product because it is useful. If that happens to attract people to link to it because of the reasons listed in the sentence before this one, then that is natural. http://www.seroundtable.com/archives/002078.html
This has been discussed before in a earlier thread before have a read here aswell about How fast is to fast? I personally dont care about getting links. I want them fast and i want much of them at the same t ime.