The usual mistake is to get too large a back size and too small a cup size. A 36B has the same volume in the cup as a 34C (this is a general rule) but a 36B on a 34C gal will be loose round the back and thus be unable to provide any support. If in doubt try one size smaller in the back and one size larger in the cup, and see if something astonishing happens to your comfort and your shape. If you are already in the right size, you are in a very small minority.

If you ever go to Europe, go to a small specialist shop (not a supermarket) and take their advice. They will be right. They may even be civil to you if the fitter is about your size ;)