0) Have an old domain, New domains are penalized thus take 6 months or more to get top for 2 word search terms. 1 word search terms can take 2 years.
1) Correct SEO, your site will be indexed properly and optimized for your targets. Miss this and miss your target.
2) Link building, this is getting harder all the time. Google see link building as the enemy and are doing all they can to discount unnatural links.
3) Remove all barriers to entry, no sign ups etc, immediate gratification.
4) Innovation and content. Killer apps that people find useful and content that is compelling.
For the record, I have about 10 popular domains, have been recently featured on Techcrunch twice and all of the suggested blogs, which are all good, but at the end of the day, search traffic is what counts in the long run.
This is precisely why Facebook and other social platforms perform so badly for advertisers, people using them are simply not in search mode. That and the adverts online are totally crap!