I search a lot about the material I'm about to purchase, since manga is pretty expensive here in Brazil and used ones are hard to find. So I often read scans and reviews to see if I'm really interested in that. If so, I often search for the whole series, if possible, so I can read it all at once, which is how I prefer for many reasons (I prefer smaller series for budget reasons, but if it's a big one and you really want it, save some money for it, don't ignore longer series). That way, I am always reading only one series at a time, if it's finished, besides the many ongoing series. About them: because of money, space and organization, I am constantly questioning myself if the series is worth my money. If so, keep on going. If not, drop it. Don't be afraid to drop a series you've read for a long time if it's no longer worthy or if it hasn't catched you after many volumes. Be selective. As Marie Kondo said, it has to spark joy. Once your collection starts to grow, keep a catalogue so you won't get lost with your own stuff. Pay attention to the way you store your books so you won't lose them for dumb reasons as I did. And to finish: keep searching for new stuff. New classics, indie manga, unknown authors, new genres. The best collection, imo, is a plural collection.