This is such an interesting thread, and really inspiring for more decluttering on my part... On the "it might be useful someday" front, I think there's a delicate balance. Plastic grocery sacks, for example--if I'm at my parents' house and I'm helping my mom put the groceries away, she's like, "Give me all those sacks. Don't let your father get any, he'll just keep them forever." And she immediately stuffs them in the trash and puts something icky on top. I mean, they have sacks of sacks in the garage that he's gotten a hold of. And you would never want to use them for anything because they're all dusty and dirty. So he would claim they might be useful someday, but actually they aren't, due to the way he "stores" them.
Always wary of this tendency in myself (I feel nature and nurture working against me), here's my own plastic bag situation. I do actually find them really useful, for garbage bags in small cans like the bathroom, for bringing lunch etc. to work, for gathering up recyclables/trash so I don't have to lug a big bag around, etc.. I keep them in a shoebox in the kitchen--nice and clean. I only keep them if they didn't get wet while transporting groceries or don't have holes. And once the shoebox is full, I stop collecting them. I'm not gonna stop grocery shopping, so I know that I will be able to get plenty more once I use up some of the ones I already have. Some people are no doubt reading this thinking, how obvious, but it actually took me a while to come up with this limit.
So I try to apply this to other things. Empty folders? Yes, useful, keep some. Don't need to keep three dozen that are beat up and torn with five different crossed-out labels on them. Old pairs of shoes, after I replace them? Keep the most recent, just in case something happens and I need them. Don't need to keep the other five pairs of increasingly old, worn shoes.
It's a tough thing, because my dad is very much a "saver" of things, and my mom almost fanatically throws things away (probably the only way the house is still livable), and I feel like I want to be more in the middle.