The first step in finding the right size is to have exact measurements. You can do this by yourself or with the help of a friend, or the best way - with the help of a bra fitter.
The biggest mistake is to buy a bigger size instead of bigger cup size. In most cases it is a bigger cup size that is needed and not a bigger band measurement. This is a special problem for ladies with big cup sizes. For example: if a Lady needs 75 E and she can't find the right size, she probably buys 85 C to get the necessary cup volume. But now the band measurement is too large for her, so the bra cannot give the necessary support. Many ladies complain about pain between the shoulder blades and the neck. This can also be caused by wearing the wrong bra size. There are also other things to consider: do you prefer a front or back fastener; wire or none wire; do you want to lift your breasts or make them flatter? For each wish you can find the right bra. Not every bra in the same size fits in the same way. Sometimes it's better to change the bra style than the bra size. Also the fit between different manufacturers can vary. For example 75 D / 34 D at one manufacturer can also be 75 C / 34 C at another manufacturer.