I'm a bit of a packrat myself, but I have two sisters, and my mother has learned a couple of ways to get us to get rid of some of our stuff.

How old is your daughter? Depending on her age, you can "guilt" her into getting rid of some of it. (Donating it to a local charity, having a garage sale etc.) Or you could try getting rid of some of her stuff while she's not around. If you get rid of some of her stuff that she really doesn't like and is keeping it "just because", then even if she notices that its missing, I doubt she can be too upset. (And if she is, you COULD offer her something that she may want - thats cheapish but that she wants or would like). Yes, that is a form of bribery, but coming from a person who just finished being a teenager, it WORKS.