NuBra - but not if you're more than a B or C cup. (They are sticky cups -they work for coverage, not for lift.)

Otherwise, you can put heftier straps on the dress and sew in some cups, or get one of those convertible low-back bras from victoria's secret. Though if you are pretty well-endowed, you might need a low-back bustier.

Have a weird dress that has thin straps too close to the neck to make a bra with straps an options, and it's too low in the back for a strapless bra. It is great in the winter with tights and a cardigan, but in the summer, not so much.

Is there such thing as a backless bra and will it stay up?