S.,
I think two things that have worked for us may help you out. First, I recommend "modelling" good cleaning habits. Usually about once a month we go though our whole house and pick out items to donate. We explain WHY other people need our clothes/things we're done with, we have everyone put things they don't want in the hallway (out of their rooms)and put them on the living room couch (from the playroom). Then together (mom and both kids) we go through the items to put them in big garbage bags. They see me cleaning out things and want to "help". Sometimes I veto getting rid of a dress, sometimes one kid wants to keep a toy another doesn't want anymore, so it stays. For a 'treat', the girls and I go to a Good Will/resale shop and I explain what "this place is". As in, see all the stuff here, it's all stuff people got rid of, just like we did. Do you see all the people shopping? Well, they need this stuff - it's not broken or dirty so someone else may be able to use it. I have used this whole process as an educational process for my children - REDUCE, REUSE, RECYCLE. (They are only 5 and 4, but they get it. ) Especially when we have a great FIND at these stores, I explain there was some "big girl" who didn't need it anymore, but because they gave away something and cleaned up, we can buy it and bring it home. I also explain that the money we are giving to buy the item goes to help people who don't have dinner, a bed, etc. I think it REALLY helps my kids to understand that there is a little girl in our area that DOESN'T have ______ (pajamas, a puzzle, a stuffed animal, etc.).
Secondly, we view their rooms as their private space. What you put in your room is yours - you don't have to share those clothes or toys with your sister or with friends who come over to play. Now, with that said, they also are expected to keep their rooms reasonably clean. (Yes, a 5 and 4 yr old have to keep their OWN ROOMS CLEAN.) I explain it as, Mom has the rest of the house you only have two rooms - your bedroom and the playroom. Now, we do have "tricks" to help them. Timeouts become room cleaning time...when your room is picked up, you can come out. Laundry is brought to their rooms, they are expected to TRY to put it away...lower level drawers for the little one. But all toys and papers MUST be cleaned up by them. If they whine that "it's too much", "I can't do it", etc. Then Mom's answer is, "OK, then I WILL." I bring in a laundry basket and a garbage bag. If I get involved it's MY RULES for clean up. I am not mean, but I explain that they have accumulated too much clutter in such a small room and that's really the problem.
As for all of the papers and art work they want to keep, we have created binders that go in their room. They love to use the three hole punch to put things in their binders...I help when the "pile" gets a bit large. I help say, "This one's ripped or crumpled, I think we can throw this one out." But quickly follow up with, "This one is beautiful, Grandma will love it." We also use some of this art work for "thank you notes" in birthday thank yous, birthday and holiday cards or when we send out new portrait pictures out to family. We've had some thoughtful friends remember the girls at Valentine's Day and Easter, so we write a thank you, include a recent portrait and art work from the kids. Everyone thinks the pictures were made JUST FOR THEM and love the thoughtful gesture.
I hope this helps you. Again, I think if you model "cleaning time" with your child, she'll be more willing to help/participate if she doesn't see it as a "chore" or "punishment" or attack on her personal items.
If this proves to be too difficult for your little pack rat, what about encouraging her to get rid of stuff and then taking her for a "shopping trip" at Goodwill or a resale shop. Like a "trade" - you help me get together a bag of toys and we'll trade them for a new toy or two.
My kids have really latched onto the idea of giving away our "baby toys". They love to think that there's a baby out there who needs a new dolly, shape sorter, etc. They really see how someone else would really like "this toy" that we're done with.
Let me know how it works out.
Sara