I always keep shipping boxes for possible returns. Then after a couple of months I break them down and recycle them. I keep a few extra around for selling things like hard drives and whatnot. I also have a giant trash bag full of styrofoam peanuts and one full of those plastic air-pocket things. If you keep any large cardboard boxes for some future shipping job, they can easily be broken down flat and taped up again when you need them.
What really piles up in my garage are the product boxes, which I keep for when I eventually sell something. Monitor boxes, motherboard boxes, chassis boxes, routers, plus boxes for all of my stereo electronics and speakers (I have four stereos set up in my home). I have dozens of these boxes. Living with all of these boxes is a lot easier than having to figure out how to pack something like a monitor or subwoofer for shipping.