Image: Her World Brides
Wear the right undergarments
A good bra or Spanx will ensure your assets don't get exposed unwillingly. Make sure it fits comfortably, and hugs every curve, so it doesn't leave a gap. For your wedding, opt for a strapless bra with underwire and a non-slip grip at the back.
If you've already got cups sewn to your dress, ask if your seamstress or bridal designer can add boning to the dress, if it already doesn't come with it. Boning will help secure the shape of the bustier, and prevent anything from slipping off, even if it may seem a little loose at the chest area.
Corsets are a great way to allow for dress adjustments (if you want to tighten your waist or top). Following waist stays, which look like bra straps at the back, corsets are another option - if you don't mind the hassle of getting tied into your dress.
Some strapless dresses have rubber inserts lining the top of the bustier. This increases friction between the dress and skin, and usually keeps it from sliding down.
Take in your waist and top
Ask your designer or seamstress to take in the waist a little more (or as much as possible without pinching your waist), so it helps support the bustier at least.
Make sure it's really fitted
Your dress fittings at the bridal salon are there for a reason. Make each visit count, especially if you have a limited number (most salons limit fittings to three sessions) of appointments to make. If you feel a certain part of your dress is too loose or tight, please let your designer or seamstress know it's a problem. Also, bring the right undergarments (and shoes and accessories for that matter) at every fitting, to make sure everything fits correctly.
See also: Gorgeous gowns for less from Rico-A-Mona
Safety pins are your best friend
If you've found that you've lost more weight even after the final fitting, check and see if your designer can fit in one last session for you. Otherwise, have your bridesmaids, or close female relative pin in your dress from the back (and affix the pins within so it doesn't look too obvious), or take in your sash to tighten the waist area.