Train up a girl in the way she should dress, and when she marries, she will not depart from it.
Dr. Marian T. Horvat addresses the importance of Catholic brides choosing modest wedding gowns in this article at Tradition in Action, which features helpful photos as well as excellent commentary. The photo of Philomena de Joras shows a great way to "fix" a dress with an immodest neckline and/or no sleeves: the addition of an elegant little jacket. This really opens up a path to great savings, because many otherwise lovely gowns are available at thrift stores and resale shops.
I think most of the problem of immodest bridal apparel comes from the lack of an everyday church dress code. A simple reminder of what is expected, posted prominently, makes all the difference. Our chapel has such a sign in the vestibule. The expectations are backed up by solid teaching from the pulpit. It seems to work for our regular parishioners, girls and women alike. Except for a few rare occasions, when I have seen a woman with her head uncovered, in pants, or a low-cut or sleeveless blouse, she has been a visitor. The same goes for the occasional man in shorts. The ones who return conform to the congregation's standards.
Likewise, at traditional Catholic weddings, the problem of immodesty (and disrespectful behavior) is generally from the non-traditional Catholic guests. They do not do so intentionally; they just do not know what the Real Presence requires of them. Some brides have addressed this problem by adding to the wedding invitation an explanatory note about proper dress and behavior.