I had a domain rank top10 for "free porn" for several years straight, which is often named among the very top all time most competitive searches.
I ended up selling it for high xx,xxx.
It was one of my first domains and I was lucky to get there as I didn't start out knowing how to do it. But in hindsight I think the main factors that got it there and helped it stick were
1) lots of varied incoming text anchors that included the words "free porn" in a longer phrase.. think there was virtually no incoming links anchored with just "free porn".
2) I had a system that saw me gaining 20 or 30 new incoming links, with varied link anchors, from on-topic sites each day. (and I never added a burst of thousands of incoming links suddenly)
3) Agreed to exchange links with many other sites who were targeting the same phrase.. no matter what their PR or competitiveness with me... as long as they were going for my same keyword. (sounds counter-intuitive to many webmasters who see all other sites in their niche as a "competitor".. but think this one is actually the easiest and most effective way to get to top of serps and stay there for your keyword... even if it does help other people too.)
4) direct recip links work just as well as 1 way links, if they are with genuinely topic focused sites. Don't get involved with linking anyone that asks for any sort of 'clever' arrangement or tries to tell you that linking you from a different site is 'better for seo'.. as it's not that simple, and these will be the people that water down the relevance of your link network.
Obviously there are other factors too... but as a basic strategy, don't just shotgun for a huge volume of random links, maintaining a majority of incoming and outgoing links relevant to your target phrase will work a lot better over the long term imho.