I keep the scraps from fabric projects (not just quilting) but I don't use them in scrappy quilts or anything like that, and I don't have a stash. I prefer to buy what I need for a project plus a little extra (in case of mistakes!) and then the scraps generally get used as "test" pieces, i.e. if I'm testing out a new technique and I don't want to use my project fabric, I use my scraps instead.
With food, I plan a menu for every week and shop for that week. I don't keep a lot of extras in the house, especially since most of what we eat is fresh and wouldn't keep much beyond a week. I don't keep a lot of canned goods and things at the moment because I hate the store bought varieties, and right now I don't have the time to make my own homemade canned goods, sauces, etc. (although I would dearly love to).
With the exception of a couple specific dishes most of us HATE leftovers with a passion, so I've gotten very very good at estimating exactly how much we need for a meal and buying only that much. It works pretty well- we have very little waste.