Kerry,
I use a variety of methods. I do have 3 Junior Library Guild subscriptions. Love them.
I use Horn Book to read book reviews and will be getting School Library Journal this year too.
I use my PLN on twitter to find awesome books, esp @mrschureads, but many others too. He posts book release calendars so you know what is coming. Many of my favorite librarians on twitter share great books they have read or blog about them. I read a fair number of blogs for nonfiction ideas too.
I regularly go to my public library, RJ Julia booksellers or Barnes & Noble and any other place where I can actually get my hands on and read the books. That way I really know whether my school community will love them.
I also take recommendations from students and staff.
I will see what topics/themes etc teachers use and make sure I have books that support them, even if the teachers don't use them, the kids will check them out.
As I am putting away books, I look for areas that we have really old books (aliens, Loch Ness monster type books were pretty ancient), so then I know to look for that kind to update the collection.
I do peruse catalogs, but it is hard to know by looking at those whether the books are a good fit.
Mainly, I read, read, read.
: ) Jenny Lussier