Are you worried about the bra itself showing when you wear the open-back dress, or just the marks from the bra you currently wear? If you are concerned about the bra itself, then bubbleloba has excellent suggestions, and there are bras that are entirely backless, as well. If you are concerned about the marks left by your current bra, I would suggest you get fitted for a new bra, as they shouldn't be leaving marks on your skin. If it continues to be a problem, try wearing a supportive camisole the day of the wedding to prevent any marks before changing out of the cami and putting on the dress.