How does anyone find a bra that fits them? I just can't seem to find one.
A few months ago I begrudginly went to Lane Bryant (I need a large band and a large cup) finally resigned to the fact that I might have to buy expensive bras to find ones that fit. I picked the one that seemed to be best (which was one band size and one cup larger than what I had been wearing) and then after wearing the bras a few days I started being bothered by the underwires. They kept poking my underarms. I guess that means the bra is too big?
It could also mean it's too small. Either way it means it doens't fit.
But at the same time, I can never find a bra in which the front part in between the cups lies flat against my chest and this one is no exception; I always have cleavage which is pushing that part of the bra out. Doesn't that mean that the bra is too small?

How can a bra be too small and too big?
Try looking for a banded bra--one with a band around the bottom. That will help keep it in place. They can be difficult to find, but not impossible. Not all bra cups are shaped the same, and it sound like that is the problem. In all honesty it sounds like you need a smaller cup size with a different shaped cup.
I was thinking of going to Lane Bryant again, and GF and I just took measurements thinking that would be helpful. Well it's anything but helpful! I found two different methods of calculating band/cup size. According to the 1st method you measure around the ribs under the breasts and add 5 to that number; the result is your band size. You then measure around the fullest part of your chest and take that number minus the band size. If you get a 1 you're an A, 2 for B, 3 for C, etc. Well according to this method of measuring I should be wearing a C up in a band size about 3-4 sizes larger than what I usually wear. There is no way in h*ll that I could get into a C cup, even accounting for the fact that as band sizes go up cups get a bit bigger. (According to this measurement GF's band size is about 6 sizes larger than what she wears and she would be an A or less, when really she's definitely in at least the B range.
It's almost impossible to accurantely measure yourself for a bra. You really need to be measured and fitted by someone who knows what they are doing. It sounds like you are a larger woman (no offense) and you might be better off going to a lingerie store that specializes in nothing but undergarments for larger women. If you don't have one around you, then check out the lingerie departments in a larger department store--Penney's, Macy's, etc. They will have a trained "fitter" onstaff. Also you might actually need that larger band. Does you bra ride up across your back? If it does, then it's not the right size or not supportive enough. A bra strap should ride between your shoulder blades, but an inch or two under the bottom of them.
I know it's frustrating as heck. I'm a larger cup size, and it can be hard finding something that fits well and is supportive. Also keep in mind that your body changes every 7-10 years, and you should be refitted in that time frame.
One other tip, put the bra on, then jump around. Wave your arms. Do a hoochie girl dance. Bend over and touch your toes. Bend to the back and sides. Pretend your Richard SImmons for a few minutes.

(Hey, I LOVE RIchard Simmons!) Then see how the bra feels. If you've "popped out" or slipped out of it just doesn't feel right, put it back. The final test is to put your shirt back on over the bra and see how it looks. Your boo-bahs will look very different once that shirt is on, and you may not like the final look.
Good luck!
Anne