Link diversity, don't spam, create steady links every day (like 100 links per day). A lot of social marketing helps too and also use varied anchor texts, don't use only the keywords you want to rank, if you are commenting try to use names as well, that will look natural too.
If your website rank in first page in search engine then you can tel that your optimization is natural..
Focus on building more good contents then backlinks will come. You can use other social bookmarking sites to get more backlinks. Forum can help build backlinks as well.