You don't have to go to a store to get a bra fitting, you can do it yourself with a tape measure.
1. Measure your chest at the biggest point, and make sure you aren't holding the tape too tight or too loose.
2. Measure your body right below the chest, making sure the tape fits comfortably.
3. Subtract the first number you got from the second number you got.
If the number you get is a _ inch difference, you are a _ cup, and the second number is your band size.
1" = A
2" = B
3" = C
4" = D etc.
For example, I would get 34" for my first number and 32" for my second number, so my size is 32B. This is the same method they use when you get a bra fitting so you don't have to go all the way to the store just to do it.
These instructions aren't really accurate. You need to take an over bust measurement and an under bust measurement and a bust measurement. Esepecially if you are triangular. For example my over bust measurement is 32 and my under bust measurement is 28. My bust measurement is 36. But I am not a 28F, i am a 32D. If your under bust and over bust are the same that is your band size. If they aren't, you can either use your overbust measurement or your under bust +4 inches. These numbers also tell you what style of bra is going to fit you best and if you are being professionally sized they can reccomend certain styles of bras to fit you.
The most important thing when buying bras is to try them on first and be able to recognize if that bra fits you.
From the HerRoom.com Fitting Guide
Perfect Fit Checklist
Let's face it, do we really know what a good fitting bra looks like? What are the signs that a bra is the wrong size? Well, let us help. Here is a checklist of what to look for.
* Smooth fitting cups: If you don't fill out the cups, you need a smaller size. If you are "overflowing" in the cups, try a larger cup size or a style with a fuller cup design. The edges of the cups should also lie flat against your breast. If there is a gap (especially with padded bras), you should go down a cup size.
* Center panel lays flat against your breast bone: If it does not, this could be a sign that your cup size is too small, or the panel is not large enough to accommodate the separation between your breasts.
* The bottom of the bra in the front and back is parallel to the floor or slightly lower in the back: The lower edge of the band should anchor below the shoulder blades. The lower the back fits, the better the support.
* A finger can be run under the front band: Your bra should be snug, but not too tight. If it's tight, you need a larger band size or a looser hook placement.
* Still not the perfect fit? Consider an alteration. We alter our clothes for a better fit, why not a bra? One woman in our office really measures to a 35 band size. She therefore buys a 36D and has a dart put in at the sides.
They even have pictures of bra mis-fits. I highly reccomend that everyone that is unhappy with their bras check it out. Wearing the right size bra makes a huge difference in how your clothes fit you. If you are going to craft up beautiful garments you do want they to fit badly because you are wearing the wrong bras.