There are many different style dresses that a girl can wear ranging from spaghetti strap to strapless. Each dress style works best with a different bra style. Low-cut dresses are no exception. It may be a little more difficult to find the right bra for a low-plunge dress because of the way most bras are made. Try different bras until you find one that fits best with your particular dress.
Decide what dress you are going to wear. Figure out any other limitations besides a low neckline. For example, is the dress also cut low in the back?
Go shopping. Try on bras in many different styles. Ask the sale clerk for help with sizing.
Try on a low-cut bra. The front middle might be cut out in a U-shape. Depending on the shape of the rest of the dress, you may want to try a strapless bra.
Put the dress on over a couple of the bras. See which bra is the least visible. Wear that bra under the dress.
Incorrect bra sizing can cause discomfort.