1. For strapless and mesh-top dresses: Strapless body suits and bustiers.
Strapless bras are great in theory, but in reality they slip, especially if you are over a C-cup. Instead, try a bustier or body suit. The increased surface area means the bustier or body suit can better grip your body and support your chest without slipping. You may also want to try going down a band size and up a cup size for a little extra hold.
2. For low-backed dresses: wrap-around bra-converting straps.
This strap, worn with a normal bra, will give you those extra couple of inches you need to pull off that slightly-too-low, low-backed dress.
3. For sleeveless and racer-back dresses: bra converters.
Not only do they help you hide your straps in the back, they also give you a little extra lift and cleavage.
4. For backless halter dresses and dresses with low arm holes: Stick-on bras with convertible straps.
The front closure helps bring your breasts together and the convertible straps give you lift that would otherwise be impossible, although they only really work on A, B, and C cups.
5. For plunging necklines: Long-line plunge bras.
Perfect for necklines that extend below your sternum.
6. For super-low spaghetti-strap dresses: strapless stick-on bras.
Some stick-on bras are made out of silicone, which can weigh your breasts down and make them look saggy, but these are super thin and lightweight. They can't do much for you in the way of lift, but they're great for keeping you covered and creating a little cleavage in barely-there dresses.
7. For backless, front-plunging dresses: Tear-drop pasties.
For C-cups and under, these special pasties use a medical-grade adhesive to lift your breasts individually and hold them in place. Sadly there is still no retail option for larger chests, although if you are really determined, you can try taping your breasts in place. Just spot test first with a small piece of tape to make sure there's no irritation, cover the nipple with a piece of fabric or a pasty to protect it, and make sure you have plenty of acetone nail polish remover and/or baby oil on hand to help you remove the tape when you're done. It's not glamorous or easy and it can be pretty uncomfortable to remove, but if you really love that dress, it's an option.