well wearing a strapless bra will make the outfit prettier and you can get some really good ones but if your not comfortable you could get see through bra straps altho that wont look as pretty.

I had a strapless dress for prom and finding a bra was a nightmare. I'm a 36 D, and strapless bras are not the best for bigger chest sizes. But I went to Victoria's Secret and found the Body (I think) Bra that has removable straps. That held up REALLY well and it gave me the support I needed. Strapless bras will slide down, but they get the job done. My dress actually had snaps and detachable straps which I wound up using later in the night. You could take a look if your dress has that, mine were in a separate baggie from the dress. Have a blast at prom!

lottsa strapless dresses you dont need to wear a bra with because they do have padding. but if you're not comfortable doing that just find a good quality strapless bra or you could go with clear straps but it can take away from the overall "formal" look. i had a strapless dress for prom and it had a built in bra. but i wasn't comfortable not wearing a bra so i went on the search to find a good quality bra, which was horrible cause i am a 34 DD. there are also those new backless bra things out to if your dress does go lower in the back aswell.