I buy things people want to check out and read. Sounds simple, no?
But seriously, some school librarians buy things because they get awards or good reviews or are classics. I'm not saying I don't buy those kinds of books, but I don't buy them because of those attributes. I buy them because someone asks me to or because I know someone (usually more than one) who will want to check it out.
I don't get a lot of money to buy books, so to get the biggest bang for the buck I make sure I'm getting things people (be they students or teachers) want.
I have a page on my library website where anyone can make a request. I also add things to my "consideration file" when I hear someone is looking for something we don't have (or don't have enough of) and I add it to the list. Sometimes it's a specific title. Sometimes it's "more superhero books."
This strategy has led to some very happy patrons. I actually get hugs and thank yous on days I put out new books. Sometime back a young man and I were discussing his obsession with the imaginary creature known as Bigfoot. He, of course, believes it to be real. No matter, I added some new cryptid titles to my list and they came in recently. I brought the new Sasquatch book over to him and he just stood there looking at it and quivering. I was about half way across the library when he regained the power of speech and yelled, "I love you!"
I yelled back, "I love you too, man!"
That's how I pick library books.