I am definitely a pack rat. A pack rat trying to be a minimalist. (Small apartment.) An interesting experiment.
I have learned not to throw away a single draft on writing projects until the final project is COMPLETED.
As for everyday stuff, how do I know what I'll need in the future? There are some cases where if there's any doubt, I keep it. Tax stuff, for instance.
As to books, that's a tough one. There was a time when I'd NEVER throw out any books. I love books. But real estate is too expensive today to keep everything. I found a library nearby that accepts book/CD/DVD donations. (Not all of them do, which surprised me.) I now periodically weed through my books and donate. I try many new authors and few rate additional purchases. Those are donated.