Wear a loose shirt with no bra. Peruse the T-shirts and try them on over your shirt and then let your shirt ride up when you take the T-shirt off. Do it right in the store, not the dressing room.
And post some pics in this thread, or share your story!
I submit the following of pudgy guy (me) only as illustration. I would love to see some of you ladies take it up!
https://i.imgur.com/PxkfuEF.mp4
Open to PM on KIK: kvdenver