One fast, easy solution for a strapless prom dress is to wear a PromTee underneath the dress.
A PromTee is simply a fancy half-tee, either purchased or sewn, that matches or coordinates with the dress. Slip the PromTee on first, then wear the dress over it.
PromTees can help with a lot of issues. Here are some examples.
Strapless or Spaghetti Strap Dress
A PromTee is also a great solution for a strapless or spaghetti strap dress:
(The black lace PromTee shown in this example was hand-sewn using the same custom pattern as shown in the previous example.)
A PromTee can fill in a low-cut dress and easily help make prom dresses modest.
Here is the PromTee that was used in the above example (this PromTee was hand-sewn, using a custom-created pattern):
When a dress needs a little more sleeve, a PromTee can help to make a prom dress modest.