My five year old niece is the queen of this!! If she asks for something at my house and I say no, she'll ask her mother. If mom says no, she asks dad. Dad works on the road and is gone at least 3 weeks every month. I think he feels guilty and gives in to EVERYTHING. And she knows this...it didn't take long for her to learn either.
I would ask her once, maybe twice to pick something up. Tell her if she cannot pick it up after being asked twice it will be taken away. This is up to you if it's permanent, such as donate to charity or just put away for a week or however long. Once she gets down to having no toys, just a bed, dresser, etc. she will realize that mom means business. She will also realize if it's something she wants to keep she better keep it picked up. If you have a kid like my niece, she has tons of toys that don't get played with. Once in a while, ask if theres something she wants to get rid of, something she doesn't play with anymore. We are having a garage sale this spring; my niece and nephew cleaned there rooms and found stuff they don't want anymore. They are selling it and get to keep the money. My niece says shes saving it for college
We'll see lol
Good luck! hope this helps!
btw, i also agree that both parents need to be on board for anything to work. Sometimes kids get confused when two people tell them to do different things, so they won't do either!