The five things you need to look out for in a well fitting bra:
The straps. They should be just tight enough for you to get two fingers underneath and pull about a centimetre up. More than that and they are too loose, less than that and they are too tight.
The cups. You shouldn't be getting the Four Boob situation (too small), or bagginess (too big)
The under wiring. Should sit comfortably in the underarm and sternum without digging into the breast tissue (too small) or crossing over in the middle/stretching around the back (too big).
The front piece (between the cups). Should be flat to the skin, not hovering above (too small) or flapping about (too big).
The underband (the bit that goes around you). Should be level all the way around, not riding up (too small) or sagging down (too big). Sometimes an underband can ride too high if your breasts are too heavy for the bra style you have, and you may need to go tighter in the back. The underband is a crucial element, you should be able to drop the shoulder straps of any bra and you bust stays in place, as if it was designed to be strapless. If it slips down, it is definitely too big.
The numbers on a bra refer to the number of inches you are around your ribcage, and the letters refer to your cup size. The most common bra size in the UK is a 36C, that's the average. Most well stocked lingerie stores will sell bras from a 28AA upwards, I know Boux Avenue sell 40H.
I hope this helps people
