My daughter always had and appreciated a little money of her own, so this was an easy strategy for us:
When she received new things, I encouraged her to decide (after the gift giver had gone home, of course!) if she wanted to return it for something else, pass it on (thus saving her an expense at the next holiday or event), or sell it on eBay and have the money instead. She made this choice before the packaging was ripped open and it became part of “her stuff.” She’s banked hundreds of dollars over the years with this strategy, and I think she evaluates her possessions more critically because of it.
I also like the “toy in, toy out” idea.