Interlinking is one of the most effective SEO techniques out there. You can use it to link to important pages using specific anchor texts.
This has a lower risk than building backlinks on keyword rich anchor texts and, more importantly, you have absolute control over it.
Whenever you add new content, make sure you link to other relevant pages within your website. Vice-versa, go edit some old pages from your site and add links to your new content, if relevant.
If you have thousands of pages, it can be hard to interlink, but you can use plugins such as Interlink Manager. Make sure you also check out this guide about interlinking.