As a kid my parents would throw a yard sale once a year where we were encouraged to sell all of our old stuff. My parents would suggest low prices to make sure everything moved but ultimately it was up to us to set the prices and we got to keep all of the money. I always thought it was a lot of fun. I might hold on to a prized toy here and there but for the most part I enjoyed selling my old toys.
EDIT: I think the trick was letting us own the process. We helped place the ad in the paper (when we were old enough), we helped design the signs to hang up in the neighborhood, we helped hang them up, we chose which toys to sell, what to price them, we haggled with the buyers (very fun), and we spent the money after (or saved it if you were my brother).
It works really well with children. Having too much crap is thoroughly depressing for them. They don't like the idea of volume reduction to start with but they appreciate it afterwards.
I've brought mine (3 girls aged 1, 6 and 9) up without strong attachments to brands, advertising and materialism. Isabella the purple unicorn has never been an issue for me.
In fact, they're rather more attached to their rain coats as that means we're going out somewhere interesting or their pencil cases as they can create something.
EDIT: I think the trick was letting us own the process. We helped place the ad in the paper (when we were old enough), we helped design the signs to hang up in the neighborhood, we helped hang them up, we chose which toys to sell, what to price them, we haggled with the buyers (very fun), and we spent the money after (or saved it if you were my brother).