For me it was link exchanging without thinking of SEO.
I run a sports betting site. But I visit a lot of other sites in that field that are interesting, have unique content that I liked.
I linked to them on the front page of my site .. not hidden away. I recommend them to my visitors as good places to go.
After sending these sites good traffic for a while, I contact them, and usually they are happy to reciprocate a link, a good link too.
It worked for me … 90% of my traffic comes through links I earned like that.