You would be surprised how often I hear this and how hard it actually is to deal with! I will start of by saying that the first all of this post will be dedicated to D cup and below and I will then add some suggestions for bigger cup sizes below.
This one is described as disposable so it wont be the most supportive and I assume you can only wear it the once. If you are over a C cup you will find it to not be so supportive but for one evening it should survive. This is by a brand called Fashion Forms which deals and specialises with finding bra solutions for dresses which i think its really good.
And the lovely people at Fashion Forms have even made a more attractive version. As far as i know it is reusable but you do have to buy the adhesive replacements.
This one I actually think is genius, again if you are above a C cup it probably wont be the best but for loads of dresses I know this will just be great.
The support on this isn't incredible and it isn't the best strapless bra on the market but its clear back does make it useful especially for things with a sheer back or things with lace on the back. It does also go up to a DD which is good.
Now what do i do if I'm over a DD?
So this is the part I would love to tell you about a brand that offers great support with strapless and backless offerings for your size, but alas, It doesn't exist (To my knowledge!) But there are a few things you can do.
If you wear a size such as a 30F that is the same as a 34DD or a 36D and whilst I wouldn't recommend doing this with regular bras it wont make a difference with the stick on ones.
This will make the back on your standard bra slightly lower, I wouldn't recommend this with a skin tight or clingy dress as it could create lumps.
So they wont offer the most support but for something just for the evening they will provide more coverage than nipple daisies.
Or the most obvious...
Go braless! Many dresses you can get away with it and if you've got it, flaunt it!