I frequent yahoo answers, and I just thought of putting one of my best answers on MommyCow:
My 6 year old daughter is very quite and loves to play with other children. Just this past year she has become a little more outgoing. The problem I am having now is, if another child that she is playing with tells her to do something she does it even if she doesn't want to. Even when it comes to adults that are strangers she lets them push in front of her or push her out of the way without saying something.
For example: We went to the zoo the other day and she went to wash her hands and she was in front of the sink trying to put the water on and a lady comes and pushes her out of the way and my daughter just backs up and waits for her to be done, Also there was an instance with a friend of hers that, the friend tells her to come to me to ask for something for her and my daughter does it. The kids know that my daughter will do anything they tell her to and they take advantage of her. I don't want this to be a problem for her in the future. I do tell my daughter when I see this happen to her that she is her own person and to not let people do that to her.
Talk to her and tell her that its okay to speak her mind and tell other people what she wants to do and tell her, they can't always force her to do things just because they're bigger or talk louder. You also need to reinforce this at first, help her find her voice. I see you have witnessed these things happen to her. Did you intervene in any way? Perhaps calling out to the adults that a little girl was ahead of them. If you acknowledge her presence politely to these people, she can see it as an example, maybe she would try to stand up for herself eventually.
I wasn't assertive as a child and always got pushed around, its hard to build this kind of confidence especially if you are a shy child to begin with. But its good if she overcomes this while she is still young.