Nancy Grace's Bra -- Keeper Of Good Luck!

Nancy Grace's bra has been a key factor during her performances on "Dancing with the Stars" -- not JUST for support, she's also using it to SMUGGLE various lucky charms onto the ballroom floor.

TMZ has learned ... Nancy's 3-year-old twins LOVE watching their mom on the show -- and they've been giving her various "special" objects for good luck -- including a barrette and a little stone.

But since they don't exactly make pockets on Nancy's teeny, tiny cocktail dress costumes -- she's been stuffing the items into her bra ... so they're close to her heart when she's doing her thing.