FYI about the post below...I'm a UK 8 on top, 10 bottom and I take a medium. I've found for the black this is perfect, the fabric isn't very thin and it's comfortable, but the purple medium I have is quite stretchy. They do give a little once worn. They are actually a lot longer than these pics. I'm 5'5 and mine can reach nearly to my knees, but I like to bunch it up slightly at the waist so it's shorter. If you like bandage skirts you will have no problem with these. Don't fear the dress! Also as Rachie commented, H&M have some identical twins of the short sleeved versions for half the price- buy them up quick!!
Right for the actual point of this post, I spent the past 4 months working in Lingerie (in Debenhams- the uniform was NICE...!) and have been meaning to make this post for about that length of time! The top questions I was asked were 'where are your strapless bras' and 'where are your backless bras'. Seems low back dresses are reeeeeeeeal popular. We didn't actually stock a backless bra, but check out Claire's fab review of a La Senza one on The View From Here! I have a clear back bra that fastens at the side with a clear plastic strip across the back but the thing is, you can still see the clear bit, and in a previous post many of you said you disliked clear straps. I don't mind them as my hair covers my shoulders 90% of the time. Plus many Debs shoppers came with 'fuller busts' i.e. DD and above, and these bras only go up to a D, BUT Debs own range is stocking them soon, if not now.
But after telling every customer that we only did clear backs up to a D cup, I made a discovery I wish I'd made a looong time ago, or I wish someone had told me! Is everyone secretly wearing these? The amount of them I threw away that came free with bras in the past is awful. You can get a low back extender (for a mere fiver in La Senza) which is just a long (plastic or material) strip which, instead of fastening both sides of your bra together, you clip one side to one end, wind it round your stomach, and clip the other side to the other end. The picture is pretty self explanatory? They are PERFECT for AA u-neck dresses as you can wear any bra you like but make it low back mwahahaha.

I don't know if I mentioned this before but I got this dress in the Topshop sale and was going to pair it with bright coloured tights and pretend to be a member of The Saturdays. But I lied to myself in the changing room and it didn't really fit, and I knew I'd never get round to altering it properly. My sister is obsessed with her birthday outfit at the moment, looking for the perfect red backless dress, she's loving all over everything backless. So she tried it on before I took it back and it fits perfectly as she has an insane bust and I don't particularly! The dress is really quite low, if you bend your elbow at your side, it hits about that low, and the converter worked great! She pulled it up a bit at the back here to show you, and she partied with it on all night in a gay bar where everyone said she looked fabulous haha. Why are we related!

Also because Debs has so many concessions, I discovered the best enhancing bras EVER! A few years ago I tried a Wonderbra and it was rubbish! They really only workif you've got lots to put in them, but if you'd love some kind of definition or cleavagee and have given up, these bras are amazing. They're called Atlantis and are by Panache, and theirlabel reads 'warning- your bust will look big in this!'. If you don't like padding then don't read on, but the padding in these really is to push up what you've got, no matter how small, as they come in an AA cup too. Seriously go try one on, put your clothes back on, turn to the side and boom, the bust you thought you'd never have! They look totally natural because you can see the definition of your real bust, rather than just padding under a tshirt. It's times like these when I wonder how many guys read here... anyway, these are great! Because of Mega Days where staff could double dip the discount, and many many sales, I've got one in each style in the end for lots cheaper. I don't know if they still have the sale on, but half the range went from around £20 down to under £10! Go now! As for the styles, the shiny t-shirt bra on the left has the most effect. It cuts really low and is full of gel, so it totally does the ultimate job. The strapless one in the centre has quite firm padding, so it's fine to wear strapless and still push you up. The third one is in the normal pattern styles which plunge and give you cleavage. I don't wear these bras daily because I'm not that bothered about looking bigger, but they are great for nights out and special occasions for a little confidence boost. Also I wouldn't recommend them for daily wear as they are very moulded so it would not be best to wear all the time.
By the wayTriumph do a 'minimiser' bra, which claims to decrease the look of the cup size for the wearer, before some of you ask again! Also Debs have their own fuller bust range called Gorgeous which is half the price of Freyas and Fantasies! But I can tell you now, I will always be jealous of my sister!
Low back extender pic from MagicBodyFashion.