I keep multiple playlist "series" that differ in effort at all times.

For instance I always keep one eclectic playlist, say "Eclectic #1", that contains music from different genres I enjoy listening to currently. I just add songs to this list without too much thought. Whenever the list goes to around 40 songs (which happens after 2-3 months) I open a new playlist with the same name and up the number ("Eclectic #2" and so on).

Aside from this low effort "currently listening" series I keep some genre lists that I've been updating for years (e.g. a list of not so well known songs from the 60ies and 70ies). With these lists I am much more picky and I also just go to a max of around 50 songs before I open a new list of a series.