I worked for a lingerie store during my college years.
Measure underneath your bust first. The tape should be even all around you and a bit snug but not too tight, it should not rise up in the back at all. If this number is on a half mark, round up. Now comes the adding...If the number is even, add only 4. If the number is odd, add only 5. Whatever the outcome...this is your band size. You need to know your band size first before finding your correct cup size.
Now to measure for your cup. First place the measuring tape over the fullest part of your breasts, again make sure the tape is even all around you. Now lean over so your back is parallel with the floor. The reason for this (and no stores that I know of even know about this nor do this) is because you are measuring ALL of the breast tissue. And you want ALL of your breast tissue to be supported in the bra, not just part of it otherwise the cups will most likely be way too small for you. Try not to pull the tape too tightly. If the number lands on a half mark, round up.
The differences in inches for each cup size (difference between the added band measurement and full bust measurement) are as follows: (And they are in US sizes)
Less than 1/2" - AAA cup or training bra
1/2-less than 1" - AA
1" - A
2" - B
3" - C
4" - D
etc....