You just need to find a dress that does not come down too low in the back so that the length of the shrug or bolero will cover. Shrugs and boleros come mostly in black or white but occasionally you can find them in other colors. A black dress with either a white or black shrug can look very elegant. A white dress with a black or white shrug also looks great. You can use either with any color dress but a great combination is a any shade of red with a black shrug.
A shawl is another way to cover up the back of the dress. Shawls come in beautiful fabrics and colors and there are even some intricately embroidered shawls that will look exquisite. It is easy to make a shawl even if you do not have many sewing skills.
You can always purchase a modest prom dresses and then take it to a seamstress or tailor to do alterations to make it a little more modest. A deep V neck can be made more modest easily. Normally prom dresses come with a generous amount of hem. The seamstress can take some of the fabric and make an insert that goes into the V neckline to make it come up a little bit higher. They can even get a little piece of lace to put in and hide cleavage.