Add songs to your library. It's given more weight than simple up-thumb/down-thumb.
That may be why my recommendations seem to work so well, unlike a lot of other commenters on this article.
For posterity, I have a little under 1000 tracks saved from 200 or so artists (of which I am following around half), around 50 favorite album playlists and about as many "I should give this album a listen" playlists.
So not really a huge collection in absolute terms. It is very much focused on hard rock and metal, though.
There's no way to know if I listened to that track in full because I liked it or because I had it on in the background while doing other work and it wasn't distracting enough (usually the latter is the case for me). Furthermore if I skip a track they don't know if I skipped it because I didn't like it or I wasn't in the mood for that style of music, or I was just sick of that song cause I've heard it enough in the past X listens.
There's no way to easily signal to spotify why I'm taking an action, and yet their algorithms are trying to make assumptions over every little action.