The essence of organizing is decision making. Clutter almost always represents postponed decisions. (So do many of those email messages in your Inbox.)
I also find that a lot of those decisions are postponed because people fear that they’ll decide wrong. Thinking it through beforehand like she did can provide a reassuring template, though it will probably need to be tweaked as you go.
One category that I think should always be considered chuck-worthy is what I call “renewable resources:” Take-out containers, yogurt/cottage cheese containers, plastic/paper shopping bags, rubber bands, twist-ties, free address labels from charities, glass jars from food, and scrap paper are things that usually come into your house without your expending any effort, and they’re all free (or free along with the thing you bought). Therefore, there’s no need to keep more than a very few on hand; they will renew themselves.