1) You can post comments on other people blogs that reference your site.
2) You can articles, that contains link to your site and submit them to article directories for others to use.
3) You can pay for backlinks on others sites.
4) You can exchange backlinks.
5) You can post on forums and have backlinks in your signatures.
6) You can hire a backlinking service - People do backlinking on Fiverr.com for just $5 (As you can see I just backlinked fiver.com :/)