For physical I hold myself to a "half read" rule. That still means a growing collection, mind. I prune aggressively but only from things I've already read, which unfortunately means I have a rump of books that I suspect are mediocre and therefore don't particularly want to read, but don't feel justified in getting rid of, whereas I get rid of books that are probably better than that if I've read them and don't think they're good enough to keep.

I don't have a solution but thankfully FiftyTwo's approach works now.Hopefully I can slowly shrink my physical pile to the point where it fits on my bookshelves.