A book could be written on this topic, but one thing must come first: Your 'On-Site' content must be of real value.
The more interesting the content, the more it will be shared. Which can lead to the links from 'relevant' directories & social media sources.
These days, spiders aren't the only ones looking at your site. There are people who'es job it is to physically review your blog or web page. My point here is: Off-Site SEO really starts 'On-Site'.
There's tons more to say, but I hope this helps.