1) The age and area of authority of the money available. It is easier to classify a site that relies more on Google or in a newly registered domain. In addition, new domains are registered for several years at a time.
2) The control of your backlinks with its own PBN (network of private blogs) and ensure that no impression in their PBN. So you want to host their websites on different ips, and save them under different names.
3) Maintain a strong anchor text profile (do not use the same keyword for all the links, which is natural)
4) Site Money on page SEO (foundation of a house)
5) A good compelling content to maintain its low bounce rate. If your visitors find your site via x-word and not immediately after landing (bounce rate), this tells Google that your site is relevant for that keyword. If people click on your site and then leaves immediately ... that will hurt your SEO.
6) Remember that SEO is: Traffic, Branding. So he has money solid funnel on site to capture leads, sales, or what you try to do.