Well as you know, I've been here before. It's not an easy hill to climb. It takes patience. The first step is to let it happen. You can't force her to pull her thong out because almost every girl won't do that. Enjoy the times when it slips out casually.
A very large part of my success is I got her to be really confident, and really arrogant about her body. She can't worry about everyone around her. The more confidence she gains, the hotter she will feel in any outfit.
One trick I used when "convincing" her that whale tails were super hot is this argument: what would you rather people see? A sexy thong just slightly slipping out of a tight pair of jeans? Or your ass crack flashing everyone? Of course she probably will say "neither" but I guarantee you she would rather people get a glimpse of her sexy underwear than her ass. I came up with this theory because my girl would constantly tuck her thong way down in her jeans which caused her ass to hang out. She didn't like her ass crack showing.
One thing that helped me is my girl, I think, was just a natural slut. She had a lot of sexiness that she loved to show off. She just had it in her head it was a terrible thing to express yourself in a slutty manor. There's a difference between: slut that sleeps with 100 guys.and: slut that shows off for her one and only guy. Being a slut can give a girl confidence, just in the right context. I wouldn't use that word alot though. Not unless she's comfortable with it. My girl likes it so that's why I use it.
Hopefully some progress is being made with you. Just keep at it and enjoy the little things.