First you need to determine whether the dress is on your body properly. This might seem like an overly simple thing but it is not that easy and this first step will reflect on everything else that happens to your dress.
The dress needs to fit onto the bust, not below the bust. There should be no more that about an inch between the arm pit and the top of a strapless dress. If there isn’t, have someone zip your dress up in back, pulling up before zipping while you physically hold your dress up in place in front. It may take two people to zip if the dress is tight.
Some dresses have a seam or band that lays below the bust. When pulling the dress up, be sure this seam stays below the bust. If you are unable to achieve both up on top and below the bust marks, the dress may be too small for you. You will want to purchase a bigger size to give you more real estate to correctly cover the bust.