Lots of helpful information here in the thread already. As mentioned previously by other posters, it's important that you first are offering the right content and targeting the right keywords; with that content being optimized for best relevancy towards that keyword. Thats the on-site, which seems that @gibintitus already went pretty in-depth there.
After which, you will start building relationships with influencers within your industry, bloggers, online groups. Once you have strong enough relationships built, it becomes easier to send them your relevant content and get links from authoritative sources. Thats the hard hustle route in a nutshell.