This is what I use on my daughter, and will use on my sons.
Right now, I regularly toss the boys toys away, but they are too young to notice...
My eldest, Drama Girl, is ten though.
We've been employing this method successfully since she's been four.
You give her the news ahead of time, and set up the rules.
You have to clean your room, and you can't come out until it's done, have fun, knock yourself out, however, if it's not done by Friday night, then Saturday we're going to do it Mommy's Way.
Mommy's way is the Garbage Bag method. I usually have to employ it once every four to six months.
It helps children prioritize between their best loved treasures and things they could (and should) get rid of.
The rules are, clean everything up in this amount of time and what ever is left on the floor gets tossed, and you don't get to be in the room....
I say, okay, and go in and empty out the toy bins of things that don't belong there, leaving actual toys in the bins. Tags, cardboard boxes, socks, shoes and t-shirts, along with empty shopping bags, get thrown into the middle of the floor. Sometimes it's necessary to throw the whole bin out but that's up to Mom.
Then I pull the red metal bin out. This is the bin where she can keep all her papers, drawings, pictures etc. I bought all her bins from target, by the way. This one is on wheels and hides in her closet, where Mommy rarely has to see it. Once this bin is full, she has to go through it, or never add to it again.
She has a predetermined amount of time to take all the things in the middle of her floor and put them in bins, toss them in the trash, throw them in the laundry pile, or put them in her desk.
Usually about an hour, two if I'm feeling generous.
At the end of this time period, Mom goes in, kicks Drama girl out (it is imperative they leave the room) and tosses everything left on the floor in the trash can.
She always asks for more time. The answer has to be no. This, in their eyes, means that you won't actually throw the toys away.
The first few times I got to throw a LOT of junk away, because she didn't believe I couldn't be as attached to her 'things' as she was. HA HA HA.
Now, I only need to throw things out every other year or so. Today, I may throw things out, because it's been so long since I've done this she seems to have 'forgotten' I am big on throwing things out.
And you never get mad at them for not finishing, and never say 'SEE< if you just cleaned this big bag wouldn't be going in the dump....'
Just say, Sorry, I wish you finished too. It sucks. Okay, well, it's clean now, sooo... enjoy!
Off to go fill up the garbage bag now....