I friend of mine, alternative spectrum publisher for 40 years, found a pretty good solution: For every book he buys, he has to get rid off another one. Literally: One book into his library, one book out.
His motivation was that he simply did not have any more space for books in his house and his SO made up this rule. When I was about 20 (already owning more than 5000 books that I had paid for with my own money) I tried to apply it. A miracle happened: After a few weeks I realized that it was not just about some arbitrary "equilibrium of books" in my bookshelf. What I did realize instead: I could not decide which book to get rid off because I could not even remember most of them! A few years later I moved out of my apartment/the country/the continent and just gave away all books I did not instantaneously decide to keep.
Ever since I stopped the compulsory book buying thing and only buy those I desperately want to read now. If I don't have time right now, it goes on a list of book to buy (compared to the list of books bought to read). Result: I read 100% of the books I buy.
EDIT: And of those I read, most go to public libraries when I am done, the few books I really, really want to keep (well, in storage in my home country/continent) fill 2 boxes. The rest of them, I can buy again when I really need them.