Waist Stay in strapless dress?

Does the dress have a corset back? Or is it just a regular zipper?

View Quoted Comment Zipper back

I would ask your seamstress, usually they can fit it perfectly to you so it stays up. If you’ve already had it fitted and it’s still falling a waist stay may be the best option. Have you already had it fitted to you?

Does it have boning? You want to make sure the boning is stopping at your natural waist and locking in at that point. That's going to help it stay on, and keep it from sagging.

Boning would be helpful. You could also buy special fabric tape to help keep it up. I would ask the seamstress what she recommends, and where it should be placed.