SEO is getting difficult. The parameters under your control are getting lesser and lesser and its all under the control of the all mighty google to judge your site. Google is getting very fussy about what links it actually counts. To deal with this I have been using a strategy for a while, its call clustering.
Under this strategy you basically make multiple sites in a niche targeting multiple keywords. You build links to them separately for a while and then link them in such a way that the stronger site in SERPs links to weak one. This allows you to have quality, relevant one way backlink.
Yes, it is a bit similar to blog farms but here you have relevant and quality sites to chose from. It works and in theory should work. Make sure you use different domain registrars, hosting and nameservers. This will ensure that google thinks the sites don’t have single owner. It works, give it a shot !