Never open anything. Never finish anything. Never take a noticeably large amount. Don't leave less than a useable amount.
This works particularly well with things that you don't necessarily need large quantities of, like milk (for tea/coffee) or, if you're feeling devil-may-care, spirits. Also, no one ever gets through a whole loaf of bread to themselves, so if you live with another person and for some reason one or both of you aren't civil enough to share bread, definitely have a few slices.
Only do any of this with terrible people who have more money than you anyway and lead terribly boring lives, perhaps in a manner different to the way in which your own life is terribly boring. Even then, only take what they won't really miss, and don't make a habit of it.