Get help from a professional if you need it.
If you are not sure that you are doing everything right, and if you want a different opinion, or simply do not want to measure yourself, go to any store or bra and underwear department and contact the seller for help. Helping clients find the best bra is part of their job, and they will know exactly what to do.
Try different brands to choose the best shape.
Not all bra brands sit the same way. In several brands, you can find your favorite bras that will suit you, but probably they will not be the same size. For example, you may need a different size in a Bali bra than in a Glamorise bra (just the way it is).