Before Winston knows Julia better, he has a strong dislike for her, and that is represented by the sash. To Winston, the sash represents
the atmosphere of hocky fields and cold baths and community hikes and general clean-mindedness.
It represented loyal Party vigor, and Winston hated the Party. It was literally an emblem of the Anti-Sex league, which recruited young women and drove a hatred of sex into their brains, making them loathe it for the rest of their lives. As his wife, Katharine was one of those types, Winston particularly detests what that sash symoblizes. It seems to indicate that Julia was an avid Party member who was completely indoctrinated to hate sex, thought, or anything that questioned the Party's god-like attributes.
However, as Winston coms to realize, Julia does not, in fact, represent any of those things. The sash is just a costume that she wears in order to fit the part of dedicated citizen. The sash, in fact, is rather ironic; it is anti-sex, anti-rebellion, anti-individuality, when Julia herself loves sex, rebels all of the time, and is a total individual in her opinions. As time goes one, Winston comes to see the sash as part of Julia's appeal; instead of inducing hatred for her, it "was just tight enough to bring out the curve of her hips," and he notices with pleasure on their first meeting that she flings the sash into the trees. It has changed for him, just as Julia has changed for him. I hope that those thoughts helped; good luck!