You know, for some of us the push to reduce, reuse, recycle is hard on us. Sometimes I have visions of this one episode of Monk where he is rewashing and resorting his glass recycling...
I do keep boxes for stuff like stereo equipment for as long as I have that piece. It's rather silly of me since I bought my house, they aren't going to be moved, and they are so old that they are long past warranty and/or the technology is outdated. I'm thinking I should push myself and that I'm going to get rid of a few cubic feet of box storage... No need to keep those awkward big boxes when I have stuff I want to keep that can go there instead!
I keep a few "handy" sized boxes around. I especially like some odd sized ones for project boxes, Amazon has some great ones that open on the wide side. It helps to store them if you can break the temptation to store them "whole" and slit the tape so they are flat.
I'm lucky that my hubby works for a plant that makes boxes, chances are if you've been to Costco you've seen some of their boxes. If we need a box, he can typically get one for me (helps that he controls the waste and inventory flow). If we have too many and our recycle box is full, he can take them to work.
When I mail stuff, I use the Post Office Flat Rate Boxes or envelopes. When I have gifts, I go to the Dollar Store if I need packaging. The rest of it I have decided I just don't want to keep around even though that voice inside me goes "oh, that's useful you better save it for a rainy day" -- well, I live in Seattle and have a lot of rainy days and still have burdened myself with a lot of unnecessary stuff.