Hi TS, take your time and slowly coax her. You can start by asking her to wear spaghetti strap top with low neck line. By showing her cleavage, it will attract alot of stares from men and eventually will boost up her confidence.
After some time, you can try ask her to use nipple tape instead of wearing a bra for outing. She might feel alittle awkward at first, but I believe she'll get used to it soon. Then slowly talk her to abandone her bra and wear those body hugging clothes when out. This will attract more stares and I am sure she'll be very proud of herself and for you too. That's how I coax my wife too
Cheers and good luck!