A friend of mine is a manager at B&T, But she worked in a specialty lingerie shop before that, and is a certified bra fitter. I would probably recommend going to a lingerie shop that displays the certificates of fitting courses. This friend is in her early 20's but started working in linegrie at 14 and is now 23, so 9 years of fitting bras, and she is good at it.

She taught me what to look for, and has told me that if I am going to get fitted, to either go to her or find somewhere that I can see that the fitters have done the training.

Look at both sides of your BBs. If the cup is not on the outside of your BB then it doesnt fit. The edge of the cup should be near your underarm, and you should not have any spillage on either side of the cup. If you have un-even bbs it is better to get a bigger size and a filler than to be spilling on one side to fit the other. 1 of my BBs is a DD and the other is a D, so I wear a booster on the side with the D cup.