With a low cut dress, your goal should be to achieve a focal viewing point from your face down through the bust line. As the old saying goes, "Hey, my eyes are up here!" To achieve the look, you want to show a lot of skin and your dress should do that. To really make the look go, however, you want the focal line to run vertically from your forehead to your cleavage. This means you need some cleavage to make it work.
The perfect bra for this look is a sexy underwire plunge bra. Theses bras are unique in that they tend to create a vertical crease of cleavage without showing any bra material. This can be done because the cups tend to be three pieces that fit snugly. They are designed in such a way that the only area where there is not a lot of coverage is in the cleavage. As a result, your breasts will naturally mold in that direction, which will create the desired look. Importantly, the plunge bra will not push the breasts up, so don't expect such a result.
If you really want to go for monstrous cleavage, a regular bra is not really going to do the job. The problem is the more you reveal, the more likelihood the bra will show. The solution is a "U" plunge bra. This bizarre looking bra gets its name from the area below the cleavage. Instead of running horizontally from the bottom of one cup to the other, the bra material goes straight down towards your belly button a few inches and then bends back up to the other cup - a "U" look. This allows you to really show a ton of cleavage, but the bra is very uncomfortable and provides little support to your breasts. It might be good for a short event, but not much longer.
The little black dress should be a part of any woman's closet. To look your absolute best in it, try a sexy underwire plunge bra. You'll be smiling at the number of looks you get.