Clothes - my new regime is I go through what they have each season change (ie 2x per year, when it starts, and stops, being shorts weather). They really don't need many clothes. If they have 6ish tops and 4ish bottoms that fit, 3 or so cardis/jumpers, socks/tights/leggings that coordinate rather than clashing, they are good to go. And it is way, way easier for them to keep on top of their stuff if it fits easily in their drawers. Anything on the point of being outgrown gets chucked unless they really love it. Anything too big or very out of season gets put away for a season or two. Then we know exactly what they have left that fits and looks nice that season, so I know exactly what else they need. I take DD shopping for that specific list. This time it was a very short list so we splashed out on individual items, knowing there were so few to buy. Before our audit I'd thought she had hardly any clothes but I realised she actually had loads, she just needed plenty of dark tights and leggings to make it all work. DS hates shopping so I get him to pick a couple of tops from online shops and get him jeans etc at the supermarket. So that is my plan really - 2 shopping days a year!
Mine have a "precious things" box each (biscuit tin) and a wallshelf to display treasures. Big plastic folder for all their certificates, and I separately have a box per child for stuff I want to keep.
I have decided DD's room needs shitloads of storage because there seem to be hundreds of categories of STUFF in her room. Books, toys, stationery, various different crafts, hairthings, cushions, her rock collection, gym & swimming gear, random clay models. So she has lots of drawers including bedside drawers, desk drawers and a trofast unit. She was brilliant when we went through it all together, she did throw away a lot and doing it with her made me realise quite how much she has. And this is a child who has few toys, really. Our current challenge is to squeeze all her new books in somehow.
We keep the topmost, inaccessible trofast box for "stuff we are putting aside for a bit but aren't quite ready to get rid of yet". I think that helps her to let go of things.
I'd also recommend a box/basket for party bag/happymeal type tat. All ours goes into that box, so all the well-loved tat is easily found and doesn't clutter up the rest of the house. Otherwise it's prime material for being homeless stuff, ie clutter.