Best Way To Build Links?

I've been working on the SEO for my site for a while; mainly guest blogging because it's a long-standing legit business and I can't afford to risk the site tanking. The content based approach is taking too long and competitors can easily get a leg up above me when they need to. I've tried the web 2.0's approach but the site slowly started to drop off - still not sure why because they were manually created, copyscape unique content, unique themes and pretty good quality and diversity links to them from GSA.

What can I do to push the site forward without investing so heavily in content?