I know this is kind of a late post, but I haven't seen anyone post about what my seamstress did, and it worked out really well for me, so I thought it might still help other ladies out there.
I had a sweetheart neckline, strapless, lace-up back dress for my wedding. I am usually a 34B but I have a long torso so if I don't wear some kind of push up bra, my breasts just look like they're hanging kind of low even though they really aren't. I was really concerned about what kind of undergarment I could wear with the dress - I'm not a huge fan of strapless bras but I wasn't sold on the sewn-in cups either - I couldn't really imagine how that would be supportive enough.
The store that I bought my dress at has their own seamstress, and everyone who works there is very knowledgeable about all things wedding dresses. So when I brought up my concern about what to go with, they all recommended the sewn-in cups. The seamstress had these foam fabric cups with a little bit of push-up padding that she recommended. She went one size up to ensure that I would have the coverage I needed as well as the lift, and that worked out really well. The biggest thing was that she sewed the cups using three or four stitches along the top of each cup, so right along the inside of the sweetheart neckline. I initially thought that she hadn't sewn in the cups properly, but when I asked about it, she said it was done that way so that the cups could move with my movement. She showed me that they were indeed securely sewn in but still able to move around, say if I twisted my torso one way or the other. Brilliant!
So getting into the dress meant positioning the dress at the proper height for my breasts, and then kind of pulling each one towards my sternum to get them in place in the cup. Then I had my bridesmaids lace up the corset and tighten it accordingly. This worked perfectly - I had enough push-up without it being too much, and danced the whole night away without any issues (at my destination wedding... lots of humidity and sweating!). It was really fantastic to not worry about a strapless bra and it was so comfortable. I would highly recommend this method for other brides!