I always remind my daughter that her stuff is hers, but since we all have to live in this space together, she needs to respect the rest of us too by keeping her stuff under control. And, until she is willing to do her own laundry and put all her clean clothes away, vacuum her floor, and feed and clean up after her turtle, that includes keeping her room neat enough I can walk through it without injury.
And I tell her she controls how others use her stuff, too. She is never forced to "share", but is free to do so if she wants, and is reminded that not sharing goes both ways.
Which leads to interesting circumstances, especially where one neighborhood collectivist-in-training is concerned. He came to the door with his (nice, polite, and enjoyable) sister a few days ago and decided to claim a candy bar he saw. His justification: "But I want it!" He didn't get it. He has also gotten chased out of the yard (by me) for refusing to respect my daughter's property and then becoming belligerent when called on it.
He is also the subject of lessons in self defense I have been giving my daughter: "Don't hit him for calling you names, only to stop him from hitting or pushing you or someone else."
I don't want my daughter growing up to think it's OK to violate the property of others, nor to excuse those who claim a "right" to violate hers. She'll have to decide for herself someday where to draw her line in the sand. But, as of now, I stand behind her decisions regarding defending her own property, I make sure she respects the property of others, and I do my best to respect her property and admit when I overstep my bounds.
It actually works pretty well. Now, if she would just clean and feed her turtle...