Have you tried a harness instead of a collar with her? Then, it is all up to you. The INSTANT she starts to pull, stop walking, and tell her to sit. Then walk to where she is, and start walking again. But as soon as she starts to pull, do it again. You will feel completely ridiculous, but she will eventually get the message: if she pulls, she doesn't get to walk. Make sure everyone who walks her does this. Be consistent, do this every walk, every day, and she will learn!