If I'm confident the channel a video comes from is likely to have nothing I'm interested in I pick "never recommend anything from this channel" or whatever the option is.
If it's a video of something like say "washing machine repair" which I know won't need recommendations of later then I right click and "open in incognito window"
If after watching a video I don't think it will generate good recommendations I remove it from my history.
Recommendations are not perfect. 3 things that would help that I wish they'd do
1) let me choose to have music separate from none music. 20%+ of my recommendations being for music is a complete waste. I'd much rather to go some special site (music.youtube.com) or (youtube.com/music) then have them mixed in with video.
2) when I pick "not interested" -> "why" -> "already watched this video" then show it as watched (put the red line under it) just like "mark as read" in email
3) don't recommend videos I already watched (this goes along with both 1 and 2. I almost never rewatch videos unless they are music videos and I don't want music mixed in with videos so the fact the 10-15% of the recommendations are for videos I already watched is a complete waste of space and time
2b) fix the UX related to not-interested. A better UX would be letting just clicking the "..." and picking "already watched" instead of the 3 step process it is now. An even better UX would be a small icon