Internal link structuring
It is a widely accepted SEO principle that linking makes a big difference to your site.
The links that point to your site tell search engines what sort of site it is, as do the ones that connect from your site to others.
Unfortunately, most of this is out of your control. One thing that is often overlooked is the links on your site to other pages on your site. They still help with indexing and content relevance. By setting up the correct internal linking structure you can maximise on the power of your site from within! This has a lesser bennefit than external link building but still provides benefit in the overall scheme of things.