Have you considered wearing a sheer blouse? Sexy and demure at the same time. Fantastic look. Sheer fabrics have been around for centuries. They are simply woven of very thin threads (of a low denier for the garmentos).
Choose an appropriate bra knowing it will be on display
Try wearing a bra with an interesting silhouette like a balconette, low-cut or strapless -- keeping in mind what serves you best. Use a bra of a similar color to the blouse keeping it tone on tone, or try the ever-appropriate nude bra.
The daring can pair a black bra with a light blouse. Gorgeous look, but very dramatic and can easily go awry.
For the more modest, wear a camisole instead of a bra. Same gorgeous look with a little more coverage. There are camis in all styles, even some with shaping ability and cup sizes.