I hear it all too often. Some bridal consultant tells a bride that, for comfort, they can just sew in some bra cups to the gown in alterations. This is almost always a mistake.
Gowns aren't (usually) engineered to manipulate your body. While adding a cup might prevent your gown from caving in, that's a poor solution. If your gown is too big through the bust, it needs to be altered to fit you. Sewing in cups is the duct tape fix to that problem.
What you want is a garment whose very existence is to give you lift, smooth bumps, and provide additional structure. Structure is your friend. Therefore, my primary suggestion for most bridal and eveningwear is a longline bra or "bustier". If your gown is quite thin, take care to ensure you select a seamless variety so that the boning of your undergarment won't show through. Most bridal gowns, however, have enough structure to negate that concern.
If you are rocking an open back gown that doesn't dip scandalously low, you can still rock a bustier. Ask for a "plunge back"! They'll generally have 3 hooks rather than the 10 or so on the regular styles. Larger cupped ladies - note that you will lose some of that support.
You say you DID go with a wildly adventurous open back? Hot! So you SHOULD sew cups in, right? Sorry, still wrong. Again, the best solution is to get your body in place how you want before the gown is on. You, my dear, need what we lovingly call "chicken cutlets"! (Silicon self-adhesive cups)
One word of caution: Frederick's and other such sites sell bridal "corsets". These ALWAYS come up too far in the back and show out the top of strapless gowns. Consider these honeymoon lingerie, NOT bridal foundation solutions.
Bottom line, a great boned, longline bustier is a foundation piece you'll find yourself using all the time once you own it and appreciate the magic. Just invest in a good one. You'll thank me. For that, and this one other pro tip:
EVEN SUMMER BRIDES: You'll want to wear Spanx (or the like) not JUST for smoothing out your silhouette so that gowns drape beautifully, but also for the wicking. YES! More layers actually means more comfort in this situation. Give it a try some warm day, if you don't believe me! Caution: if your shapewear clings to your gown, you may need a slinky slip in between.