I have a stretchy black lace bandeau that I wear under dresses like this. Bandeau is better than a cami because (1) no shirt under your dress to add bulk or get all twisty, and (2) no dueling necklines or straps - it just goes straight across so stays hidden no matter how wide your dress neckline is. So I think it looks tidier and more formal.

This isn't the exact one I have, but it's similar.