I have to be really careful with this, and it's a royal pain in my rear. I have the mixed blessing of having kids with creepily phenomenal memories and strong attachments to their possessions
I stash toys in the storage closet (I call it purgatory, got the idea from a friend) and wait to see when it gets asked about again. ds1 has remembered things after not playing with them for two years and has had complete meltdowns over toys being missing. (not saying this is normal, btw, I'm not exactly sure about that...) I've been purging ruthlessly this summer and just two weeks after a huge yard sale dd asked to play with something I had gotten rid of. boy, did she ever have a wholehearted cry over that!
telling them they can get money from selling or trying to pitch the charity idea goes over like a lead balloon, too. ds1 FLIPS OUT if I even suggest it. so, I sneak stuff out in black garbage bags when no one is looking; I've used childcare from my MIL to load my car; and I've said "I don't know where that is" which is technically true. I feel guilty about some of the things I've gotten rid of (items I hated that they didn't), so mostly I try to stick with items that I feel pretty confident they really won't miss (as long as they don't see it going out the door!
). and, like I said, I still have screwed up some this summer and have been yelled at and cried upon for it!