It’s actually a good question Joseph. I wish I knew the answer.
I know of someone who had some kind of incentive program for the teenagers in her neighborhood. It must have been at least somewhat successful since I heard about it from one of these teenagers who had a lot of hakaras hatov to her and told me about something the teenagers were doing to show their appreciation.
Granted, this teenager was not dressed tzniusly (although I don’t know how she dressed before this), but the fact that they felt positively about it was a good sign.
I think that most of these teenagers need for it to be given over in a positive way, which is why this was a good idea.
I don’t know what could be done for adults. I know they have some kind of organization that organizes meetings for women to discuss and learn about tznius. I was a bit skeptical when I heard about it, but then someone who was going to them told me about it, and I had a better impression of them after speaking to her.
I think that most people who have issues with tznius nowadays need some kind of “support group” to work on it, since it clearly is not simple for them. That is probably a good idea for any middah that people have a hard time with and need to work on.