The dress... :)
This is an elastic tape that I found at the fabric store, which is commonly used for strapless garments. It has a piece in the center that clings to your skin, helping the garment stay put.
Pin and Sew
You don't even need a sewing machine for this step, it can easily be done by hand.
I decided to also adjust the zipper for a better fit since the dress was a bit big for me.