J.C.
J.,
I think the other poster is right; what you do is going to depend on her maturity. If she's not going to understand what you're saying, you should just skip it and non-emotionally take the object from her hands and wait for her to grow a little bit more.
If you think she may understand some reasoning, you could try what we do. I started by saying "Honey, we share in this house". And then I would very conciously play with toys and then give them to her, saying the same thing. "Here, Honey! We share in this house" The big thing was that I made a point of picking two or three really special toys that she DIDN'T have to share. She knows that her leapster or her Cinderella doll are off limits to her little brother. But everything else is fair game. That way, when my little girl starts fingering my jewelry box, I had something to fall back on that could be mine and not sharable.
Now that she's three and understanding reasoning and a little empathy, it's a little easier. On playdate days, when we have a quiet moment before our friends come over, I sit her down and we have a talk. "Remember when we went to Max's house and he shared all his toys? Didn't that make you feel good and happy? How would have felt if Max wouldn't let you play with his toys? ..." etc etc. And again, I take two or three toys away from sight so that they are hers and only hers and not in the playdate mix.
Good luck to you. I know it's frustrating, but it gets easier, I promise!
J.