1. Sizing principles
There's a lot of confusion about what cup and band sizing mean, and their relationship. Essentially, the band size is the size of your rib cage, and the cup is the difference between the band and the fullest part of your chest. The sewists amongst you will probably be familiar with the D cup as 2" difference, but this isn't strictly true when we're fitting a bra.
Using my favourite pub discussion (men love to hear about this for some reason) we can illustrate bra sizes.
Imagine I take a standard sized bottle top, and place it on the front of a coke can. It's quite big in proportion to the can right? So the coke can lady probably has F cup boobs, with a 32" band. Massive right? Now take the cap and place it on a large bottle of coke, suddenly that's tiny. Our bottle lady has A cups on a 40" band, despite her boobs actually being the same size.
2. What size band?
It wasn't so long ago that the smallest back size you could get was a 32", and the largest around 38". Many women, therefore, got pushed into whatever was closest to their size.
The rule that gets flug around is usually chest measurement plus 4-5" is your band size, but actually I think the same issues that affect the rest of the clothing industry with vanity sizing means that this isn't really true any more. I'd always start off looking at the current brasize and how it fits. You should always buy and wear a bra on its loosest fitting. That allows you to get a bit of extra grip by tightening it as it ages and the elastic loosens. On that fitting, the bra should be firm around the chest - it should be able to stay up on its own if you take the straps off your shoulders. I find most women can go down one or two band sizes - the tighter it is (within reason - you still need to be able to breathe!) the better support it'll give and the more comprotable it should be.
Something else to look at is the space between the cups. This should sit flat on your breastbone, so that the band hugs your rib cage all the way around. The band should also be level - the same height at the back as the front. Most bras that are too big rise up towards your neck in the back. This is because the bras aren't giving any lift, so women tighten the straps to hoick themselves up - you shouldn't need to do that if your band fits properly! In fact, the straps should be the last thing you adjust to give you a little lift after you know the rest of the bra fits well.
3. In your cups.
Now you can look at cups. This suffers the same problem as band sizing - because cups A-D were standard for years, many women wear whatever was closest to their size, even though they needed an F! The cup should sit comfortably all the way round the breast tissue, not digging in, bagging out or wrinking. At this point it's worth trying a few different brands, as they're all cut in slightly different shapes and will suit different people. Personally, I know that Freya, Fantasie and Rigby and Peller usually suit my shape well, but other brands do have individual bras which work well too.
If you're getting bulging over the top of the cup, you need to go up a size. The rule is generally that for each band size you drop, you need to go up a cup size. Like the other rules, though, use this as a guide to start from!
Don't be afraid of the larger cup sizes - you don't need to be built like a porn star to wear a big cup - to illustrate this point - I bought a 30H last week, and I'm not porn material! My measurements are around 38" full bust, 30" underbust.
Below is a good exaple of a terribly fitted bra. We all know Christina Hendricks has big knockers. My guess would be around a 30-32" F-G cup. The bra she's wearing in this photo is probably a 38D. You can see clearly the the cup is sitting right on the tip of her boobs, and the wire must be cutting in terribly. The wire should run below where the horizontal seam is, and about half the distance between the strap and her back. You can clearly see from the shadows that her boob continues well below the line of the cup and goes right back under her arm. I'd guess that there's been some significant photoshopping done here to make her chest look smooth and perky, because she'll never get that look from a bra that size!
4. Shopping for bras.
My first piece of advice is to do this with a friend. There will be a lot of chopping and changing of sizes and brands and the last thing you want is to keep dressing and undressing to get another size. Shops with good bra fitters can save you a lot of work, but beware those sales people who use a tape measure, and anyone in a shop which has a limited range of sizes - they'll only know how to fit you to their closest size, so you may end up with something that only roughly fits.
Start with simple designs - once you know your size in a brand, you can try the pushups, strapless and other specialist designs (and bare in mind this the size might change slightly for fit in some shapes).
Don't forget to have a good jiggle - lean forwards and tip yourself into the cup, get your hand in and move everything around - you need to give yourself a helping hand to get everything in its right place!
If you think you're getting in the DD-E cup territory, you should probably find a shop with plenty of larger sizes - in the UK your best bet is Bravissimo. I say this because once you stray out of regular high street sizes, you need to give yourself the chance to find a range of styles and brands that go up well above your expected size. Don't limit yourself to the 'plus sizes' rack of the department store!
Don't forget to look properly at your size every couple of years, and if you gain or loose weight.
A properly fitting bra will not only make you look better and make clothes fit better, but it can help your back and shoulders if you're very large, and avoid the damage from badly fitting wires, which isn't pleasant and has been linked (though I can't find any actual studies to support this idea) to breast cancer.